'rocket hunter' is a simple little radio transmitter that sends out a peroidic beeping on a certain RF band, you wander around and listen as it gets louder. they dont really work over a mile though, and they would probably get smoked by buildings and crap in the city.
i've put some thought into this too,. and up untill recently i just rode shitty peds nobody in their right mind would steal. i'm getting a little more freaked about it now.