He'd noticed that some personal items were missing and, even though he had no idea who was responsible, jailhouse rules dictated that he had to ask his bunk-mate to "strap up" - put his shoes on for a fight.
The small, pebble-shaped gadget, which can be hung on a keychain or clipped to a belt or purse strap, vibrates, lights up, then sounds an alarm when users step more than 30 feet from their phones.
The app itself offers a somewhat grainy but definitely workable image of what the camera is seeing, letting you line up the perfect shot before you strap into whatever exotic adventure you're about to partake.