Tamara Pilblad sat on a green bench in the park. She was reading The New York Times – or, at least, pretending to. I sat down next to her. There was no need for pleasantries. We both understood why we were on the Lower East Side that morning.

“The subject’s over there,” Pilblad said, gesturing with her head. I didn’t look. It was my first stakeout and I wasn’t going to blow it by gawking at the subject.

“It’s OK, you can look,” Pilblad said.

I turned my head and there, about 30 yards away, was the nanny. She had a full, curvy figure and straight brown hair that fell over her shoulders. She was talking on her mobile phone and pushing a baby in a swing.

I turned back to Pilblad. She saw the nervousness in my eyes. “Try and relax,” she said.

It was easy for Pilblad to relax. She was a licensed private investigator, a professional nanny spy, and this was just another day’s work for her. Pilblad is the founder of Whereismybaby.com, a nanny surveillance outfit whose services include GPS tracking, background checks and nanny-cam installation.

It’s a good time to be spying on nannies. In the past year, Pilblad’s business has doubled, and she now installs an average of nine video systems a week, typically rented out for $100 a day with a three-day minimum. One of Pilblad’s competitors, the NY Nanny Cam Company, claims a 50 per cent increase in business since 2000. And Joseph Brocia – the owner of Security Zone International, the company Pilblad buys her equipment from – says that he’s seen a dramatic rise in systems purchased for this purpose.

“In the past it was, ‘My employees are stealing,’ or ‘I want to watch my warehouse,’ ” Brocia says. “Now, I would say the vast majority [of customers] are parents concerned about the caregiver.”

When I ask him about the rise in nanny monitoring, Brocia points to the high-profile criminal cases involving nannies, the increasing number of working mothers with small children and the falling costs of the surveillance equipment.

Another factor may be parents wanting to make sure that nannies are earning their salaries. Top New York nannies make upwards of $15 an hour and a recent survey of daily nannies in London found that their average annual pay is now almost £30,000 a year (up 6 per cent in two years) with live-in nannies’ salaries averaging £22,500 (up 12 per cent in two years).

A few months before the stakeout, I had interviewed Pilblad in her apartment on New York’s Upper West Side, where she lives with her seven-year-old daughter Lily and her dog Cornflower. She is 37 and petite and has a soft, slightly raspy voice that sounds subdued until she begins to talk about why it’s important to keep tabs on nannies, at which point her entire body becomes animated.

After graduating from Georgetown University Law School in 1997, Pilblad spent five years in the US Army’s Judge Advocate General’s Corps, where she rose to the rank of captain. She then moved to New York and, after a stint as a labour and employment lawyer, took a position in Citigroup’s ethics and compliance division.

At Citigroup, Pilblad saw at first hand how a large company monitors its employees, right down to reading private e-mails and instant messages. “Every job these days has a built-in way of monitoring employees, especially the high-stakes ones,” Pilblad said, her voice picking up steam. “The highest stakes for me is the wellbeing of children, but that’s where we have the least amount of safeguards and protection.”

But what really motivated Pilblad to start a nanny surveillance company wasn’t what she witnessed at Citigroup but what she witnessed at ballet class. On Fridays, Pilblad would leave work early to pick up Lily after dance class. Most of the little girls were fetched by nannies, and while Pilblad waited for the class to end, she would sit in a small room with the nannies and listen.

Some of what she heard was troubling: nannies talking about dropping off their charges with other nannies and going shopping; nannies visiting boyfriends or taking babies to their own apartments in other boroughs. And some of what she heard was horrific: one nanny admitted that she had given a child a big dose of a well-known cold remedy so that she would sleep all day. Another claimed that she used a gas stove to get the child to go to sleep, figuring that since she weighed a whopping 300lbs, and the child a mere 35, she’d have plenty of time to turn the gas off after the little girl was asleep and before it knocked out the nanny herself.

Pilblad said that some of the nannies spoke only in Spanish, and I can’t help wondering if she had misunderstood the gassing story. But whatever she heard, it resonated with her. She had had her own bad nanny experience when Lily was two – a nanny had asked for several weeks’ pay in advance and then disappeared.

It was with these episodes in mind that Pilblad took an exam to become a licensed private investigator and set to work starting her own nanny surveillance company, first incorporated under the name Merry Pop-ins, in May 2005.

Pilblad told me that she had wanted to hide video cameras in her apartment to see if I could find them, but a client had rented most of the cameras the day before. Instead, she led me through the rooms and pointed out all the places a camera could be.

“See that globe on the cabinet? It would be easy to punch a camera through there,” Pilblad said, pointing to one of many quirky knick-knacks. I followed Pilblad into her living room, where she pulled a book – Chinua Achebe’s Things Fall Apart – off the shelf and showed me how she had taken out the insides and made a hole in the cover for a camera. In Lily’s bedroom, she picked up a teddy bear. “I could put one in here, but I don’t like to use teddy bears. Kids like to touch them. A clown doll is better. Kids are scared of clown dolls.”

Pilblad tapped a portable hi-fi on Lily’s bedroom cabinet.

“Is that a camera?” I asked.

“No, but it could be,” she said. “At the spy shop they sell a camera that looks exactly like it. You can’t tell the difference.” I asked Pilblad how anyone has time to watch all the footage from their nanny cams, especially people such as her clients, who have multiple cameras set up in different rooms. It seemed like someone would have to spend all night watching the footage. Pilblad didn’t seem to see the problem. “You can watch four cameras at once on a split screen and fast-forward through the slow moments,” she said.

We sat down in the living room where I noticed a suspicious-looking Rubik’s Cube on a stand on the coffee table. “I bet that Rubik’s Cube is a camera,” I said.

“No, that’s a Rubik’s Cube,” Pilblad said.

“What about that antique phone?”

“That’s an antique phone.”

I asked Pilblad if she ever felt guilty about her spying. I could understand why some parents felt the need to do it, but I couldn’t entirely rid myself of the feeling that it was wrong to watch another person without his or her knowledge. “I tell parents that the best thing to do is tell the nanny before they hire her that she’ll be monitored. It protects the nannies too,” Pilblad said. “People are people and nannies are human. They have lives. They get upset. They make mistakes.”

Pilblad explained that the cold remedy and the gassing were extreme cases. The bigger problem was the well-intentioned nanny having a bad day and lashing out at the child in a moment of stress. “People are people,” she said again.

I scanned the room for more cameras. Pilblad disappeared into her bedroom for a moment, then returned with a box of baby wipes. “Look at this,” she said. She opened the box, removed a thick layer of wipes and pulled out a rectangular GPS tracking device. “You can put this in a nappy bag and then track your baby on a map.”

Usually, the cameras and GPS devices are enough to answer parents’ questions about their nannies, but sometimes there is no substitute for live nanny surveillance.

Our stakeout day was one of those times. We continued to wait in the park. The nanny pushed the swing. The baby swung back and forth. “Do I look OK?” I asked Pilblad. I had thought a lot about what to wear to the stakeout before deciding that my usual attire – jeans and a short-sleeved, collared shirt – would be sufficiently inconspicuous.

“Your clothes are fine, but that big bag you’ve brought with you isn’t helping,” she said.

I wanted to kick myself. Probably somewhere there is a rulebook for spies, and on the first page it says: whatever you do, don’t show up for a stakeout carrying an enormous blue bag. I knew the bag was a bad idea, but I needed to bring my computer – box and all – to a nearby store for repairs.

“Try and relax,” Pilblad said.

I asked her what made this baby’s mother suspicious of her nanny. All Pilblad knew was that the mother had been having doubts about the nanny for some time, and now she wanted the hard facts.

Pilblad took out a folder and showed me an e-mail the mother had sent her. The mother described the nanny, “a little thing, but curvy”, and the baby, “pale-skinned, blue-gray eyes and chubby cheeks – not much hair yet”. Then the mother listed what she wanted for the $50 an hour she’d be paying for Pilblad’s services (a rate that has since risen to $85).

“I pretty much want to know everything: What time does she leave the apartment? Where does she go? Does she put her on the swings? Does she ignore her? Is the baby ever crying? Is she on the phone the whole time? Does anyone join her? Does she stop for food? Is the baby sleeping at any time? If so, where? On her, in the pram? Does she feed her anything or give her any bottles? If it’s sunny, does she have her hat on at all times? Exactly what time does she get home? Having more experience at this than I do, you may be able to suggest other things to note.”

Ten minutes passed.

“OK, let’s move,” Pilblad said.

I looked up. The subject was leaving the park. By the time I picked up my bag, Pilblad was off. I ran down the street to catch up. She was on her mobile phone. “She’s walking toward Rivington Street,” she said. “Hurry.”

“Who was that?” I asked, still struggling to catch my breath. “Matt,” Pilblad said. “He’s driving the van.”

The van? There was no time for questions. The nanny was keeping a quick pace. We stayed about 20 yards behind. Now and then Pilblad took a pad of paper out of her purse and jotted down the nanny’s co-ordinates.

At Ludlow Street, the nanny turned the corner. We were suddenly in peripheral vision. Pilblad hooked my arm in hers. I wasn’t sure why. Presumably she wanted us to look like a couple in case we were spotted, but it wasn’t clear that looking like a couple would make us any less conspicuous. I decided to play along, hoping no one I knew would see me walking arm-in-arm with a woman who was not my wife. The nanny turned another corner. Pilblad stopped and looked up at me.

“Kiss me,” she said. Her voice was urgent.

I looked into her eyes. Was she serious? I didn’t want to ruin the mission, but I also didn’t want to jeopardise my marriage so that Pilblad could keep her cover. I pictured myself telling my wife what had happened. “The good news is that I helped a nanny spy keep her identity secret. The bad news is that I had to make out with her to do it.”

Pilblad, meanwhile, was moving ahead as if nothing unusual had just happened. I caught up and we followed the subject down another block. The nanny turned again and Pilblad looked up at me. “Kiss me!” she said.

I saw then that this was not a joke. I tried to think of something, anything, to say. “So the idea is that you don’t want her to have a chance to see your face, right?”

“I have to use whatever resources I have,” Pilblad said, already heading down the block.

The nanny went into a cafe and Pilblad and I sat down on a nearby bench. I wiped the sweat off my forehead. I had expected to feel guiltier about spying on someone than I did. It may have been that I was too caught up in the action. Or maybe it’s harder to feel guilty now that we’re all being spied on all the time. Last year, the Royal Academy of Engineering released a reported revealing that Britain has 4.2 million closed-circuit cameras and that a typical resident is caught on camera an average of 300 times a day. The US may soon catch up: the New York Police Department plans to place 3,000 cameras in downtown Manhattan in a plan modelled on London’s security system.

When the subject left the cafe, we tracked her three blocks back to the apartment building where the mother and baby lived. Matt’s van was parked in front of the building. I stepped into the passenger seat and Pilblad scooted into the back.

Matt sat smoking a cigarette in the driver’s seat. He had long wavy hair streaked with grey and wore black jeans, black cowboy boots, and a black leather waistcoat with a barely visible tank-top beneath. Had a team of experts travelled the world in search of someone who looked like a 1980s rock star stuck in 2008, they couldn’t have done better. Every touch was perfect, right down to the dirty bandanna hanging from the knob of his indicator.

Matt told me he was a carpenter who worked with Pilblad on the side. He had a New York accent. I asked him if he enjoyed the spy work and he launched into a short speech, somewhat less articulate than Pilblad’s, about why it’s important to keep tabs on nannies. “Someone’s gotta watch out for the kids,” he said.

“Matt cried once,” Pilblad called out from the back of the van. I looked at Matt. “I didn’t really cry,” he said.

“Remember that one time when we saw the nanny shake the kid? You were tearing up,” Pilblad said.

“OK, I was tearing up,” Matt said.

As he spoke, Matt toyed with a rubber chicken that was attached to a long pole. The chicken pole was a prototype of a device Pilblad had invented to attach to prams and buggies. The idea was for the chicken to stick out in front so that oncoming traffic would see it before the pram appeared in the street.

Pilblad reappeared between Matt and me with a cool-bag full of snacks. I took a bottle of water and some salt-and-vinegar crisps. The conversation drifted off. I tried to think of something to offer. “It’s a shame that I didn’t bring a disguise and pretend to be a delivery man,” I said. “I could walk right into the apartment.” I added a slight chuckle to indicate I was trying to be funny.

“Why? Were you hoping to chloroform the nanny and rape her?” Pilblad said. “Ummm … uh, no,” I said. “I was just thinking that for all this work following the nanny, we don’t have any idea what’s going on indoors.”

“The mum’s got nanny cams,” Pilblad said. “She knows what’s going on in there.”

The van was sweltering and Pilblad said that if the nanny stuck to the schedule the mother had given her, we wouldn’t see her again for several more hours.

I decided to drop off my computer and then go to my office, only a few blocks away, to cool down. When I returned a few hours later, I ran into Matt hurrying down the street. “She’s on the move,” he said under his breath. For the first time, I felt nervous for the nanny. Pilblad and I didn’t make the most intimidating duo, but I wouldn’t want to discover Matt was following me.

Up the street, I saw the nanny stop and adjust the umbrella on the pram to keep the sun out of the baby’s face. The poor woman, I thought. She’s taking perfectly good care of that child and this is how she’s being rewarded. Then the nanny, who had apparently claimed she spent afternoons with the baby in the park, pushed the pram inside a New York University dormitory and didn’t emerge for the next two hours.

I have no idea what the nanny did inside that college dorm, but it didn’t seem like a good place for a baby.

Pilblad waited in the van, and I stood next to Matt on the corner. The sun beat down on us. Matt didn’t say much. My exciting stakeout didn’t feel so exciting any more. The weirdness of the moment – I was stalking a woman and a baby with a man who looked like a member of Status Quo – was no longer obscuring the depressing story beneath the surface. We had somehow arrived at the point where overburdened parents were hiring strangers to watch their children and then hiring additional strangers to monitor the first strangers.

It was hard to blame the mother for worrying about her baby, and in keeping tabs on nannies, Pilblad really was keeping children safe. The real problem was one that no one could solve – the problem of the modern world and its countless obligations colliding with our primal need to help and protect our babies.

When the nanny finally left the dormitory, we followed her back to the apartment. And when I saw her adjust the baby’s umbrella again, I began to wonder if perhaps she’d had a perfectly good reason to be in that dorm. The guilt, long overdue, finally hit me.

At least temporarily. When I checked in later with Pilblad, to ask what happened with the nanny we’d been watching, she told me that on day two of the stakeout, she had caught the nanny smoking a joint in the park. People are people, I thought. But then: And some of them shouldn’t be taking care of babies.