Header Tank Overfilling (still)
Lets first start by saying I am not familiar with your system. I am only visioning it from your description.
With that said, I can only think of two places your mystery water would be coming from. First expansion. How close to the overflow is the water when it shuts off. Could the water be getting enough warmer when not in use to expand to the overflow? Second possibly I can think of Do you have any single valve faucets or shower valves in the home. The kind were both hot and cold are controlled by a single knob? Maybe pressurized cold water is seeping past seals into your hot water system (kind of like an internal faucet drip) If this is the case it would most likely be a minimal loss but could overfill the tank after long periods of time.