I had one of those come off before, it's a cap to one of the pulleys for the alternator or waterpump or that is what it looks like. Yours looks like it has parts broken, which is why it came off. I just bought a new one from BMW around $5.00, but mine looks slightly different from yours, but close.
A forum community dedicated to BMW owners and enthusiasts. Come join the discussion about Bimmerfest events, production numbers, programming, performance, modifications, classifieds, troubleshooting, maintenance, and more! Bringing the BMW community together.