I had serious battery drain. First I thought my old battery died so I replaced it. After 4 days, that battery was dying, I had to charge it every other day. Finally after 2 weeks that battery crapped out and they replaced it under warrenty - dead cell. Tried all the FSU tests, FSU seemed ok. Began testing for parasitic drain, when I found the 'hood open' limit switch unplugged. I had done brake work last month, checked the brake fluid, and forgot to plug the limit switch back in. That was the source of the drain.
Sometimes I am my own worst enemy.
1995 525i 5 speed - sold at 225,000 miles
2002 530i 5 speed - 187,000 miles