David,
I'm sure this is just the coolant gradually filling various areas of the system which have airlocked. My 1993 +8 does exactly the same thing, and it can take quite a few hot/cold cycles before coolant remains in the expansion tank on cooling down. This is especially so if the "heater" (a euphemism if there ever was one) has drained down. I've never tried a water wetter, but I'm sure that someone on here will be along soon to describe their experience with it.
Dave
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