My two dogs must have been secretly trained by an escapologist as cunning as Houdini. They regularly escape from our home and fenced-in back yard and trot back exhausted hours later. Meanwhile, we have no idea where they are or what they are doing.

More seriously, some families have elderly relatives suffering from dementia who may become disoriented and lost, or young children they want to keep tabs on between home and school or, perhaps more controversially, teenage drivers who may ignore speed limits.