This may give you an idea of what others have experienced:
Alarming Situation
In my case it was the motion sensor. Until they could get the part, I had to turn it off when locking the car. (Push the center dash button with the toe hook on it. It turns off the motion sensor, but leaves the rest of the alarm functional. If this "cures" you're problem, you've at least got a pretty good idea of the source of the problem when you take a to the dealer.)