I've never understood the point of car alarms. Things that set them off appear to include: leafs falling off trees, cats, thunderstorms, and people walking past the car.
And as a result nobody ever pays any attention to them, including the owners of the vehicle. Car alarms go off all the time around here and I have never. ever seen the owner of a vehicle appear to investigate - presumably because they also know it means nothing.
All they do is wake me up at night. (Them and the police helicopter buzzing around for ages for no discernible reason - wonder if I could call the police to complain about noise pollution by the police?)
Given that in this country its actually illegal to sound your car horn while stationary I don't understand why car alarms are even legal, as that's essentially what they are. (Granted the only time I've ever heard of the car horn law being enforced was when someone sounded their horn at a police car in front of them, but its technically a law).
Even house alarms don't seem much better, as there always seems to be at least one house in the street with one that goes off every other day for no reason so again you get used to ignoring them.