Coolant Loss !!!!

[Klaus]

Well, they are right - The coolant has to be going somewhere!

It's either leaking on the ground or in the cab (heater core), leaking internally into the engine oil or the combustion chamber, or leaking into the transmission fluid. The only other possibility I can think of is that the throttle body has a coolant line running to it to keep it from freezing. If the throttle body got cracked somehow I suppose it could be leaking coolant into the intake charge.

Given you've already had a problem with the spark plug, I'd say that was your first clue. I'd pull the new spark plug out and take a look. Compare it to another plug pulled on the other side of the engine. If the plug looks bad again I'd say you either have a blown head gasket (if you're lucky) or a cracked head or block.