If you are losing coolant and can't really find it, I would think the head gasket is another possibility. We just got out Ford minivan back out of the shop for this very problem. The van never did leak that we knew of, although my wife drives it 99.9 % of the time. I always check the water level every month or so, and last time I checked, it was ok. A week or so after I checked, the wife says the heat is not working very well, so I check and it is out of water in the radiator. By the way, even though it was really low on coolant, the van never never showed that it was getting hot, must have had just enough to cool the motor, but not enough to heat the interior.
Maybe your engine is getting really hot and that is what you are smelling? Anyway, just trying to help.