Re: It's drinking coolant... The mind boggles.
If you really can't find any external leaks then you should get the cooling system pressure tested pronto..this will a) Show up any external leaks you may not have spotted b) If there are no external leaks and the pressure drops..then its head gasket I'm afraid.
You should get it investigated before doing any real damage to the engine. Had this on our little Skoda and it turned out to be radiator on the way out and a slight drip on the water pump. Can't complain after 114,000 miles