There is no sensor to detect if the car gets rocked/moved. As far as I'm aware, the alarm system is quite simple and I believe it will only go off/activate if someone tries to open the door from the inside if it's locked.
As Red suggests, look at the little black plunger on the front slam panel - they are very prone to being broken off.
Also, it is known for the micro switches in the doors to get stuck/fail causing alarm and locking issues.