It supposedly is a sealed system but at the same time I have heard that it is not. I believe the consensus is that it should not lose fluid. I do know that our expansion tank is under pressure which is different to many cars where the expansion tank is a plastic bottle like the wiper fluid (flip cap and all).
I top mine up every other month or so, not much loss, dipstick is just a bit lower.
2006 330i - Jet Black - 19" E46 M3 wheels - HR Sport - 199,000 Miles...