That is probably true. Could it possibly be a leak in the heater core itself?

 

Been dealing with this since I replaced the HG in the beginning of January. It gets less and less each time. I'm not nit showing coolant anywhere, oil, ground, no over pressure in the system, pressure test shoes nothing. I never lose any real amount. I tend to keep the level at the he seem line. It has never fallen below the full mark. I'm guessing persistence is key. Just keep plugging away.

 

hmph. sounds like you've really worked the problem. Have you checked all the hoses? I once had the hose that connects-up at the firewall ballooning when hot, then would let air in. didn't see it 'til like the 3rd or 4th time bleeding the system.