I'm not quite what you are saying.. but I've heard that each cop car broadcasts a radio signal that reaches about 1 mile. And if you had a police scanner tuned to it, you can tell if a cop is close by.
This is a quote from another forum:
"I have a Bear Tracker scanner. Not in my current car, don't panic Teutonic Purists. It mounted under the dash and is about the size of a CB radio. I think they have updated and made it much smaller. I automatically scans all normal Police, DOT and Highway Patrol frequencies. It has a special little part that scans only ultra low wavelength repeater transmissions used by Hwy Patrol cars to let you know if you are within one mile of one. They don't even have to transmit for you to pick them up. Pretty cool."