Ok wife's 2003 300m. Two years ago it over heated and turned out to be thermostat that swelled up to point of not opening. Anyway been driving it I bet close to 25k since then with no issues. Monday she calls me says it's in the red n chimmed it was over heating. Hauled it home put half to 3/4 gallon antifreeze in it. Been running it around here and it's not getting above half on gauge. Ran car with air on high for half hour to point fans kicked on high. Checked oil no water in it. Appears no oil in water. Fans turn on. Top hose is hot. Bottom hose is cooler so water pump and thermostat I'd say are working. Checked temp on upper and it's like 200 when fans are on high and lower is like 125. Hose coming out of overflow tank is like 170. No leaks that I can see on radiator. Doesn't leave any signs of leaks in garage or underside of car. No white steam or smoke our tail pipe. Doesn't have antifreeze smell inside car or wet floor so I'd say heater core is ok. I'm stumped without doing leak down test I don't know. Any other ideas? Been watching fluid levels and haven't gone down. I got the overflow tank marked and it stayed on that mark since did the thermostat before. What am I missing?