Coolant Level Polo 9N 1.4TDI - Help!

Can anyone comment on the following situation?
Last Friday, I had the coolant topped up after driving 38,000 km. The level had been slightly above the minimum from the beginning (or at least, that's what I believe). I had it topped up without any warning lights coming on, just as a precaution. After the refill, the level was even slightly above the maximum. Today, I checked again, and the level is now only slightly above the minimum again?! Where did the liquid go? Or is the level somehow related to the temperature (expansion?)? I doubt it could be that much!
Can anyone help me? I'm a little worried about my partner!
Since the level has been consistently slightly above the minimum before, I don't think the head gasket is blown. It's also very unlikely to fail after only 38,000 km!

Hello, don't worry.
Okay, unscrew the coolant reservoir cap, and the level will probably be back up to the maximum mark.
If not, then fill it with water and check it again after a thorough drive.

http://www.cheesebuerger.de/smiliegenerator/ablage/7/467.png