another reason for false car alarms
 

I had parked my 2003 X5 at my friend's home in dense city. I was sleeping in the guest room and the familiar alarm went off. I could use the key to reset the car and this happened twice before I just unlocked the car (I could do this from the bedroom) the next day a spider desended from the visor, right by the motion sensor. I grabbed him and threw him out. Problem solved. No alarm the next night. The alarm had gone off unpredictably over the past few weeks and it is now silent.

I have had a couple of those alarm-go-off-for-no-reason moments... I will look for spiders now. :) (mind you, in my case the windows are up all the time.)

This is also a known trick by car thieves. They set-off your alarm few times till you get tired and unlock it for them.

spider wasn't the problem
 

The alarm went off again. I searched this site and found the solution. A post about the hood switch for the alarm. I just sprayed some contact cleaner in it and wiggled it some, and then glued a thin piece of cork to the hood where the switch makes contact. No alarm problems now for a week. Thank you X5 and all the great posters. Rich