I am having the same problem with the '01 VW Golf TDI. Was reading through this when I read about your alarm going off. Apparantly mine was having this problem b/c when in the shop for the battery (both at the VW and local shops) thay said the car would be locked and the alarm would go off all of a sudden with no near it. It turned out to be a bad door sensor and was reading that it was opened or being opened while locked and would set off the alarm. (Only reason I didnt know was because I dont lock my car in the garage.) They believed that to be the issue draining the battery, fixed it, but unfortunately that wasnt the case but hope it helps you if you still have the car or have the same issues with another.
