[clf]

another thought, that caught me out. Was the cap tightened fully? I felt I was overtightening the cap, so put it on hand tight, and then a little further (still by hand). It was only 4 days later I could smell that fearful smell of hot coolant, walking past the car. The level had dropped enough to empty the expansion tank over the four days! (that would be roughly 80 miles). No obvious signs of leakage either.

Refilled, bled to be sure and tightened until it reached the stop (still by hand but with more effort). All was good after. I still replaced the cap, even though it was only about 2 years old anyway.