Invisable Coolant Loss? No visable Leaks?

sounds like a head gasket, cracked head or cracked block, pull the spark plugs, are they really clean? head gasket then, the coolant is going out the exhaust, most of it is burning up before it gets to the end of the tailpipe, unless your using water instead of coolant, in that case the water is dehydrating from the heat of the engine.