I know I am resurrecting this, but just found this vid which explains how to check the coolant. What the guy in the vid is saying, is what I'd expect based on the diagram on the coolant reservoir - which is a little different than what has been said in this thread for the most part.
Basically, when cold the coolant dipstick should pop out of the reservoir to indicate things are fully filled. If it is just flush with the top, you could stand to add coolant.
I noticed my radiator fan was going nuts (and it is not even that hot). Checked the level and my float is below the threads right now. Guess I'll add a bit of distilled water, can't get any bmw coolant until Monday.