The car system used here in the U.S. is called "Lo-Jack," as in the opposite of "hijack." I don't see how putting the transponders in ignition keys would do much to stop theft, but maybe I'm unaware of European developments. (There are keys with chips in them, acting as electronic keys, or to make the keys harder to duplicate, but not to track the cars.)
The computer which controls the engine checks whether there is a transponder in the ignition key. The key sends a 32 bit ID number. If this number was registered in the computer, the engine works, otherwise not. You can't start the engine just by shortcutting some wires.
Perhaps a drug dealer may be more usefull if he moves free and has a transponder inside which he doesn't know about, that having him in jail.
[ This should have been "than having him...". I was very tired yesterday evening. It's embarrassing to see how many typos I made :-( ]
Implausible. The theft detectors are not picking up specific transponders, just the "on" or "off" state of the things attached to clothing, books, CDs, etc. (I say "things" because some of them are strips inserted in books, some are tag-like things clamped to clothing, etc.)
No, the theft detectors don't. But they are big and unsuspicious enough to hide specific detectors for big brothers...
Again, the infrastructure is lacking. The simple detectors in stores would have to be upgraded to track more sophisticated transponders. The stores would have to cooperate, etc. Implausible.
Why implausible? Stores cooperate. I know about a big department store in Germany (but I don't tell you which one) which has a large secret military hospital and a medical stock below it's basement. None of the employees knows about. If they cooperate in having a complete hospital inside, why shouldn't they cooperate in having some antennas and some wires? Hadmut