Seems like a common problem.. I have had this issue twice now. Dealer just put in my 3rd new battery today, but this will NOT solve the issue. I insisted on finding the root cause, which seems to be impossible judging from all the above posts.

There must be some electrical device(s) that keep on pulling power from the battery, even after the 15 min shutdown period. Otherwise, the battery drain cannot be explained. I have no hot wired connections at all, I never play the radio with car shutoff. I always lock the car when its parked.

Seems to be much harder to find the root cause because of all the electronics in the car. A shame that BMW did not test this all the way to the end...

I also told my dealer that I do not want to have my car back until they have solved this once and for all.

Will keep you guys updated !