You don't say if it happens at a particular time or place, say after a drive, in rain etc. I've not had any problems but I did read in the BMWCC mag a letter from a chap whose alarm would go off 30 mins after arriving home. It was suggested that air was being drawn through the vents and causing it to trigger. Close vents, prob solved.
Never heard of anything like that myself, not sure if it applies. I think I'd start by checking/cleaning the switches on bonnet etc.