Do you see anything dripping, or is there anything wet either on top or on the underside of the vehicle? Do you see white smoke coming from the exhuast? Based on everything you said it sounds like a (badly) blown head gasket. But if so, the engine should also be running very poorly, with the check engine light blinking and a misfire code. And I would also expect even a really weak mechanic to find that problem very quickly, which makes me think it's something else. Were any other repairs done recently before this started happening?