Same idea but with Bluetooth instead of nfc would be cool. The other Bluetooth device could be anything from your headsets (that you keep in a different pocket) to your boyfriend's phone when you'r out together.

The only issue is... What exactly will happen when the Bluetooth connection is lost? Play a loud sound? The thief will simply switch it off. Keep sending text messages? Coordinates? Pictures taken by both cameras? Is the idea patented?