Pressure creeping up is only an expansion issue if this happens as the water heater burner or elements are ON, heating up an infusion of cold water. It sounds like this is NOT your case.
One question is this: Which side of that check valve is the expansion tank located on? Try turning off the pump and see if the problem recurs. If it does, it points to your PRV, and these do go bad. If the problem goes away with the pump off, that would point to an issue with the location of that check valve.