I had exactly the same problem with my 2.8l GV (2007)...it turned out to be the amp in the back draining the battery quickly and causing the alarm not disarming, then main battery discharging and then alarm going off (activating) at random times *presumably when the main battery is about to completely discharge. Alarm only stopped when the auxiliary battery died. Hope this helps someone in the future! (Cost me £500 in tests and diagnosis etc)