It seems, I'm having the same issue now.
Last night, it was the first time my E39 was ever left parked outside overnight. Our garage is full of stuff because, we just sold our home and we're moving out next week.
It is a very quiet neighborhood and this morning with no apparent reason, the alarm just went off. I unlocked/locked the car with my remote and few minutes later, the alarm went off again. I again unlocked/locked the car and the alarm went off few minutes later.
I'm going to check the battery and hood sensor. Also, Is there a way to disable the motion sensor?