They do make GPS collars for pets that track their location and other useful data. Not sure of the range, though.
Collars, obviously, would be useless against kidnappers. A few years ago an urban legend in Mexico caused kidnappers to cut up their victims in a pointless search for locater chips. They were too stupid to realize that no such technology had yet been invented.
Apparently the rumors started with news articles about chips implanted in employees that would grant them access to the companies facilities. A type of key less lock. The chips will not locate people, however, at least not yet. That would take an implantable GPS, an implantable Cell phone transmitter, and an implantable battery with a multi year useful life.