Two small, battery-powered units that communicate solely with one another via radio frequency. Each unit is bluetooth enabled, and can be programmed via your smart phone for any desired distance (up to whatever the max of the RF signal is). Both units feature audible alarms. When the units break the desired distance, both alarms go off, and can only be turned off by either bringing them back in contact with one another, or via the tethered smart phone.
Clip one to your baby, and the other to the guardian. No more leaving babies in hot cars. No more kids wandering off at the super market. No more public abductions, without you, and everyone in earshot of the kid, being notified. In addition, if a unit breaks the distance, it could send a kind of “alert” signal via RF that could be picked up by anyone else with a smart phone – kind of an instant Amber Alert. This signal could then be used, via the cell phone network, to provide an exact and up-to-date location of a lost or abducted child, so long as the unit remained with them.
I guess kids aren’t the only application, either. Strap one to your dog’s collar at the park. Hide one inside that briefcase full of blood money you’re supposed to deliver to those mobsters. The possibilities are endless.