I've never had this problem, but a buddy with a 91 318is had the exact same problem, turns out the door locks are also heated, when you pull up and hold on the door handles, it turns them on. As it turned out, they were staying on all the time, thus draining his battery after his car was turned off. He ultimately had a ex BMW mechanic find and fix the problem.
Just another possible area to look at.
1998 323is Track toy