*could* be the headgasket.
I presume you've done the usual checks by removing your oil cap and looking in there for mayonaisse-like substance??

Make sure you go round and physicaly check every water hose as a leak may well be enough ot empty it over a couple of days. Is the floor wet under the engine bay? Another sign of external leaks. If the floor's dry, then more likely to be an internal engine leak