[white 125M]

Probably nothing to worry about if as you say the car is parked or sitting nose down but I'd would park the car on level ground, wait until the engine is cold and then check the actual level by unscrewing the coolant cap and checking the (built-in) gauge. If it's down, then top up it with clean tap water or demineralised water. If you don't want to top it up yourself, get your BMW garage to inspect for any leaks. I think I read elsewhere in this forum of one or two people having small leaks due to improper fitting hoses.