The coolant level in the expansion tank of my 140 Sigma has always gone up and down depending on use. If I use it as intended it goes up and driven gently it goes down. The system has been pressure tested and is fine. It's a mystery to me and my best guess is that air is somehow being entrained at high revs, trapped somewhere in the system then released upon more gentle use. If anyone has a better idea I'd be interested to hear it.