There was actually a good DC31 talk called "Snoop On To Them, As They Snoop On To Us" kinda in this vein, but with Bluetooth devices that are part of a lot of cop's gear.
Some people I know are building a similar system, watching for the printers that parking attendants carry to issue tickets. When they see one of those nearby, it starts the clock so that they move their car before the time expires.
https://www.youtube.com/watch?v=cO1JSzAdPM8