Hmmm. Well, the laws of thermodynamics says it has to go somewhere. Some options might be:
1) Physical leak. I'd look first at the oil filter gasket where it meets the engine, the oil pan drain plug (loose?), and the oil pan itself. It could be the RTV seal is leaking between the pan and the engine, or that a small hole has rusted through your oil pan. If the leak is small, it might only drip when pressurized (driving) so you might not see a puddle of oil on your driveway.
2) Burning oil. A bad head gasket or worn piston rings might cause this. But gray-blue smoke when accelerating would be the main symptom, and you said your exhaust doesn't smell like oil.
3) Oil in the coolant. Another option with a bad head gasket is that the oil pressure could force some oil into the coolant passages. If so, your coolant level in the bottle will rise, and it will become sludgy, like a coolant/oil milk shake.
Sounds like you're looking at the right things. It makes me wonder if the head gasket repair was done properly. Let us know what you find. I'm also curious if there are other options I haven't thought of.
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AtTheHelm Front Recovery System & Roof RackM116A3 Trailer build thread: viewtopic.php?f=72&t=77997