quattroworld

Almost like the system is getting pressureized?

First thought is potentially a head gasket, however oil shows no signs of coolant/contamination, and coolant is still bright pink, so no signs of oil infiltrating. Not to say it can't be a head gasket, just no other tell-tale signs. Coolant tank doesn't "hiss" when I open it after driving.

Both hoses are hot so thermostat is operating as intended. Could a failing/failed after-run/secondary water pump create enough pressure or backup to cause the coolant level to rise in the coolant tank, but return to normal after sitting?