Hi, how's it going?
Have you found out what that part is yet? http://www.realoem.com/bmw/partgrp.d...52&hg=11&fg=45
Are there any part numbers or makers name on it?
There should be a bleed screw on the thermostat housing, this should be left open when filling the system untill you get coolant with no air bubbles.
There are hoses at the back of the engine for the cars heater, it could be leaking from there.