[andrew_h] has put together this slick anti theft device for his car. The RFID immobiliser is used to keep the car from starting unless you swipe an RFID tag. Depending on how well you hide it, and how well the person stealing the car knows you, they would have no reason to suspect that they have to swipe the tag. Even if someone steals the car while it is already running, they won’t be able to re start the engine if they shut it off. As usual, schematics and PCBs are available.