I have seen a setup using a boat foghorn (way louder than your average car alarm) to keep people from stealing gas. Switch on the gas door would set off the foghorn when the door to the gas cap was opened unless disabled by a second switch in the cab. Friend with this setup said he had gotten a couple of free gas cans and siphon hoses using this method, evidently just donated to him as they were left in his driveway.

Might be able to do something similar using the spring wire that holds the emblem on. Be sure to place said foghorn as close to the item to be defended as possible for maximum effect. If you are not where you would hear the foghorn (at work?) I would suggest a timer to prevent a dead battery.