[jocko]

Just resolved similar issue in a newer truck - some antifreeze smell, no puddles, no overheating, but lost noticeable amount of coolant on a 1 hr drive (in the current TX heat). Newer trucks have the stupid plastic tanks, and I had a hairline crack that expanded when hot and let coolant drain out while driving along, but when I'd stop and get out and check for leaks, zip. Crack wasn't obvious, hard to see until the radiator was out. I realize you don't have plastic tanks, but what I'm getting at is that I think lutronjim is probably hitting the nail on the head. It's worth a close examination of where your core meets the tank - a crack or leak there will give you the combo of symptoms you're seeing.