The usual contenders are;
The header tank cap - Unscrew it carefully while the engine is just a teeny bit warm - you should hear a hiss, just before scalding hot water spurts over your hand, so wear rubber gloves, use a cloth and don't stick your face right next to it obviously
Heater matrix leaking - Feel about underneath and see if you can feel wet patch of carpet inside the car. Look for blue runny marks on the heater matrix where water may be seeping out.
Loose hoses - Check the tightness of the hoses. Again, the blue stains test might point you in the right direction.
I had to tighten my (already tight, I thought) hoses, to stop a long-running water level drop. The system is pressurised, so a minor leak is unlikely to leave drips under the car. More likely the waters nipping out just when the engine is hot and it's under pressure.
Cool heater may be down to a air lock, thermostat problem or just becuase the water level is so low at the time.