You could just do what I do and leave a totally insane blue heeler in your car. If anyone gets with in 5 feet of his dart he starts barking and goes nuts.
A friend of ours had his German Shepherd with when he stopped at a store on the way home. In the 2 minutes he was in the store, the dog managed to wiggle out the pick-up window and disappear.
I called the cops and let them know we were looking for a lost dog and gave them numbers to call if it was reported.
A few minutes later, they called back with a report of a German Shepherd sitting in a convertible near the store. The dog wouldn't leave and was "protecting" the car from anyone getting close to it. Ray went over, only to find his dog sitting in someone's car. However, the car was the same year and color as as the Olds 442 he had at home. He even had a hard time convincing the dog to abandon her guard.