Just methodically inspect the entire head gasket perimeter.

If still flumoxed, add UV dye to the coolant, and use a matching UV light to hi-light the leak point(s).

Eitherway...you should be able to find it.

If you lose coolant, but not oil...and not TO the oil...it would be like mine...coolant to the engine exterior.

My leak was the drivers side as mentioned, but more to the rear of the engine, facing the back actually...tucked WAY up there and hard to see.

A jar of stop leak, and it was cured.

Well, for a few thousand miles, including to Canada and back, etc...and then I hydrolocked the engine and swapped in a new engine altogether...so the stop leak never failed at least.