Seeing as how I am going to do a train or semi horn in my truck, that is a nice idea. Except when you accidentally set it off XDi have seen a 120Db train horn hooked to a security system. when it was tripped, the pump would come on and within 5 seconds open the horns and they would gradually ramp up as pressure built. it was a custom rig tho. i can only imagine the thief's reaction when the truck starts to whir at him then whistle.
and yes. he said it has gone off before and has had the cops speak to him for noise violations. he said he asked the cop how many times he actually responds to car alarms going off.