I once had a very slow leak (~1pound/week). It drove me crazy for months. I would check tire and refill weekly. I took it to a tire store and even went through a mechanical inspection in NY. Nothing! The tire guy said it might be a leak from the wheel or maybe the tire wasn't properly seated. Well, long story short - there was a tiny nail imbeded in the tread. Got the tire patched, end of problem.
What I'm getting at is the dealer may have looked for the source of the leak, but not found it. Stuff happens!
Oh, and yes, a BMW dealer will patch a tire...ask me how I know.