This post seems kind of hysterical. i doubt the car alarm go off when a hat drops onto the car.
Car alarms are fickle. There are sensitivity settings for car alarms, but ultimately it depends on the type of car alarm. There can be volume settings, but again, it depends on the car alarm. There are a lot, and I mean a lot of car alarm manufactures.
Some alarms have just off and on settings, others have more features.
Its not uncommon for an animal to set off an alarm when it jumps onto the vehicle. Even loud noises (such as a exhaust backfire or high revving car) can set them off. Again, it depends on alarm.
A car alarm is great when someone is about to swipe your car. Alas, its so not so great when it goes off due to a false alarm.
Im not going to doubt your cousin who installed the alarm. Because the alarm is doing its job. Its not my place to say.
But if it is really bothering you, I would recommend with alternatives.
Some alarms out there had silent features when it tells the remote that unauthorized access has occurred.
Some clifford car alars have these features. It's one that I recommend.
www.clifford.com
Another type of security measure is a lojack system. Its a type of tracking device mounted somewhere on the car. So when you report the car missing, the police can hunt it down asap. It's often referred to as "lojack"
www.lojack.com
Anyways. I hope this was found useful.