Again, ALL RFID tags, by their very nature, need to be inexpensive. At a certain expense, it is cheaper to install a commlink. I'm pretty sure that at commlink pricing, that is an added feature, along with security monitoring, which the end user pays for and subscribes. If this is something that all (registered) cars have, then the cost needs to be negligible (or included in the registration fees). City wide locating of vehicles would be done via the transponder tracking. A stealth system would likely require a hand held wand type device to confirm a vehicle ID. So, lets say a cop rolls up on a suspicious car. The transponder says one thing, but he decides to check further. He pulls up the RFID code from the database, plugs it into his scanner, then waves it over the car looking for the appropriate ping back. Since he doesn't get a ping back, he starts checking the stolen vehicle list and running those codes. That is enough of a time sink that a cop isn't going to run those codes if he gets a matching RFID signal with the transponder. Our current day equivalent is a cop checking the plates and then checking the VIN on the dash. If the VIN matches the tags, then he's going to accept it as legit. If it is missing the VIN or it doesn't match the tags, that's when he'll push harder.
The trick then is to go to the junk yards and find wrecked cars with the same make and model. Grab the transponder and RFID code. Get those registered legit and when you steal a car, plant those numbers in the car. If you want to feel good about yourself, you can say that you're being Data Green and recycling codes that would otherwise get lost into the junkyards forever.
