All the heat is dissipated whilst moving but when you stop you have a chunk of metal that has to get rid of the heat, usually through the surrounding air and mostly through the coolant and radiator so you will get a temporary increase in temperature until it all cools down. So if your header cap is missing you could get an actual boil up (a pressurised system will boil at a higher temperature) and you will also get an increase in the coolant expansion and the header tank overflowing.
So always wear your cap.
