The coolant system *should* be a closed system. They used to be designed with a pressure bypass that if the coolant gets too hot and there is too much pressure in the system, it will blow the extra coolant into the holding tank. When it cools, it sucks it back. My GM truck has the tank intergrated fully, with the cap on the tank being the pressure cap. Not sure how the 3-series is though (I'd assume the latter).
That being said, I have had to add coolant once to my car at about the 40k km mark. Mind you, I've never checked it, so who knows if it was even at the top when I took delivery.