Most likely the heater core. When heater cores go back, sometimes they start out with a pinhole leak. The water evaporates quickly so you never see a wet area.
A sure indication of this is uncontrollable window fogging. When it's cold enough to use your defroster, the car slowly fogs up even with the defroster on full blast. And when you clean the windshield on the inside, it never stays clean and has a film (ethylene glycol) on it.
Been there, done that.....