Hi Carla,
The most common cause for the alarm going off is the alarm sounder unit, which has it's own battery. When that battery goes flat (because it's old and doesn't hold it's charge), the unit think's there's a current drain (like a door opened) and sets off the alarm. If it's at the same time, is it, say 6 hours after you park your car?
If you trawl the internet, you'll find a lot of people with similar problems on Zafira's and Astra's too, with the solution (new sounder, or just remove it!)
Rob, August 2008
