I had a slow, "fillable", coolant leak this summer. For a couple short trips I filled the reserve tank to compensate for the leak, but any time I got the rig up to temperature, I'd see the leak under the rig after I parked, presumably directly below where the compromise existed, and the leak would only really occur when the system was at pressure. Once the engine had cooled off and pressure had returned to atmospheric levels, the leak would disappear. But the reserve tank always told the true story because it was always low on the old marker when the engine was cold.
I refilled my overfill tank with straight coolant (older 460 V-8, so old tech), just to make sure I didn't run low or overheat. I added much less than a half gallon over the few weeks I had to run with the leak, which shouldn't have messed significantly with the coolant/water ratio, much.
Took the rig into a mechanic, and after a rather expensive total radiator replacement, no leak, as it should be.