Designer Dennis Siegel's RFID Bikealarm should be enough to ward off any bicycle thief, or at least scare them considerably. Siegel's alarm emits a shriek measured at 120dB whenever it detects abnormal movement, aka someone trying to steal part or all of your ride. The alarm is placed below the seat and activated by scanning an RFID tag, with a small beep telling you the alarm is armed.
Siegel's RFID Bikealarm was his part of his senior project for his BA, so it's missing a few frills, such as smartphone connectivity. Still, it does the job quite well with its "limited" functionality. A loud noise should at the very least be able to draw either your attention to your bike or someone else's attention to the thief. You can see the RFID Bikealarm in action in the video, here.