I had the problem of the alarm going off when I was parked on a slight downward slope, and passing traffic would generate enough road vibration to set off the motion detector. Once that was disarmed, it was fine.
Also had the problem last week in the middle of the night as my battery was apparently going dead from an unknown/undetected drain. Finally had to leave it unlocked in my driveway for a few hours until I got up and recharged the battery.
2000 528iT sport (Sienna Red w/ Tan)
2000 540i/6spd (Sienna Red / Tan)