One thing you can try - lick your fingertip and then wipe it on the windshield above one of the defroster vents - then taste it, if it is sweet, you have a tiny leak in your heater core.

Check around the reservoir, the radiator, and any hose connections - a small leak will dry, but leave a residue, often a white powder

If removal of the cap on the reservoir causes a release of pressure sound after a night, and your engine is stone cold - you can't have much of a leak.

How are you topping off the coolant?

I suggest that you only fill it to the MIN line when the engine is stone cold.

Check it every morning to see if it is going down. A pain in the neck, but it does not cost anything.

We had an '89 745 GLE that would not stay completely full. It always went down to the mid point between MAX and MIN between services,

My dealer would top it off at each service and a few days later out would go down to the mid point again.

Eventually I drew a line at the mid point and marked it "full".

The techs never topped it off again and it never lost coolant for the next 4 years