With the engine up to temperature the water will splosh out at you when you undo the cap, because it is under pressure.
Try running the engine up to temperature, with the header tank cap off and see what happens. You might be able to see it "drinking" the coolant, or you might have the beginnings of a head gasket failure and the pressure is not exceeding the cap's rating yet.
Just so you know, it took us several weeks to confirm that a head gasket had gone on a Mercedes. It had incredibly mild head gasket failure symptoms, that gradually worsened over the following weeks. Eventually it began to leak water out of the header tank's overflow pipe, until we found the cap was past its best. Replaced the cap and it stopped leaking out the overflow.
However just a week later the rate of water consumption increased. If the engine was revved to 2,500rpm it would only then generate clouds of steam, at idle it was barely noticeable.
So be weary of a head gasket in its early stages.