All the rules etc are no problem, I am just looking for ways to actually detect/track the vehicle. I have been looking into RFID but it looks like getting a reader that is powerful enough to read a tag on a vehicle a couple of meters away is going to be far too expensive. I guess one option could be a cheap android smart phone (< $100) in each car, hooked up to the 12V supply and hidden away behind the dash. This could run something like Mqttitude which automatically posts location updates to my home server MQTT broker. This seems a bit of an overkill though.
What about a simple bluetooth transmitter? Can you buy these standalone? I already have a Raspberry Pi in the garage which is controlling my irrigation system, so I could chuck a BT dongle in there and maybe use that to detect when the vehicle BT TXs come in range?
Any other bright ideas out there?