Originally Posted by 740iS

If the cap was leaking, and you're using BMW antifreeze, you should have that tell tale white residue from after it dries. While I agree your cap could be faulty, unless you have residue on the top of the expansion tank, I'd say you have the well known slow leak from the bottom or side of the tank.

When my tank went bad, it was in a location that I couldn't see any residue. I just knew I was having coolant leaving the system and decided to try the tank first. Especially since it has been mentioned on here a lot too.