Rented a pressure tester and...
Engine not all the way cool, but my first test (if I did it correctly) points to air escaping from the neck of the overflow tank just below the lip. It looks like either someone already tried a reapair with some clear sealant around the neck or it is disintegrating a litttle.
If it is true, would it be possible to re-seal the plastic enough to hold pressure. Would a small pressure leak cause coolant loss even on the overflow? After driving the car, then opening the cap, there was some pressure built up.
Look for another leak, maybe? No coolant on the garage floor.