Forget about the alarm system. Now I'd be really worried what else this hack and his amateur buddies experimented on during their weekend garage-booze sessions. I'd get really well acquainted with my car if I were you so that you dont realize during winter, the car just placed water instead of EthylGlycol and your car keeps heating up or better yet he figured a way to turn down the mileage before selling it!
You got to anticipate this stuff and be alert- most people here will tell you stories of the stuff you'd find hard to believe but happens everyday.