Kühlmittelverlust und die Werkstatt findet nichts | Posts 16+

I hope you're right.
In such a situation, there's nothing else to do but follow the rules of the game.
At the very least, we can now assume that this is not an isolated incident. Four days and 500 km driven, and the expansion tank is empty.

Purely subjectively, I have the impression that the water disappeared anyway during the cooling process.