Was your sunroof open or windows cracked? Did something maybe trip your alarm? It happened to me a few years ago. Many other members have posted the same thing. I think it was chalked up to the alarm getting tripped. It didn't reset until I got out of the X, locked it (alarming it), and unlocked it (disabling alarm).
Here's my thread:
http://www.xoutpost.com/bmw-sav-foru...s-instead.html