Last weekend I met up with some friends, both of whom have BMW 3-series sedans (type E46). Upon arrival, one of them started leaving a puddle of coolant in the parking garage. Not good.
I could see it was dripping out from one side of the undertray, but the source was otherwise hidden. We filled the radiator with water and my friends took the car to the nearby BMW dealership where it was discovered the coolant expansion tank had developed a leak.
The repair quote was $400 at a minimum to replace the tank, though it was unclear whether there were further issues since the car appeared to overheat somewhat on the way (I wasn’t there). The videos below go into a more detail about this model’s particular issue:
As an aside, the other friend’s BMW also suffered a broken headlight the same day due to extremely windy weather combined with an unmanned shopping cart that made a beeline for his car. It was not a good day for Bimmers.