When I installed a used Nissan diesel in place of the ailing Nissanb engine, the difference in the cooling passages was stark. I had to exchange most everything marine cooling so I got a good comparison.

The new Nissan which was regularly maintained, including flushing the cooling system(3-5 years?), showed completely free cooling passages. Conversely, the engine I took out(and I didn't exchange the coolant at all in 10 years...) showed a white build up around all the cooling passages. I would guess the flow of the coolant was reduced by-guess, 20%.

It's possible that the engines cooling temperature could rise slowly over time, unnoticed, and maybe extra-but small expansion space is needed in the loop-which is slowly forced out either pressure cap?

All long shots, but if you take your water heater out of the loop, while it's open, you might as well flush the system and add new anti freeze. If you do regularly do this, then, never mind,...