Most alarms won't trigger just from somebody running a key down your paint. the problem is if you install a motion sensor and it is sensitive enough to trigger from a key running down the side then you are going to constantly be dealing with false alarms. Its too bad that somebody did that to your car. Unfortunately probably the only thing you can do is get your insurance to pay for it and move on. If anything, a security camera setup near the location of your car (if possible) would be your best bet.