Car alams usually work by monitoring a voltage drop. So for instance, when you open the door the internal light comes on, hence setting off the alarm. So check you havent anything that is drawing power, such as a courtesy light left on.
The sensors inside work on "pressure", so movement moves the air which changes air pressure.
I'm afraid I cant be of more help than that. Apart from saying, I remember a post ages back on here about alarms going off.
Hope this is not completely useless info for you.