An expansion tank also has cold and hot fluid lines that should be periodically checked, and one can observe true radiator fluid level without removing the pressure cap.

Radiator cap Hose going to the coolant overflow tank, a hose that goes from just below the radiator cap to the overflow tank Coolant overflow tank ...

If it’s not obvious, you can find the overflow tank by following the small-diameter hose that is attached to the radiator’s filler neck, just below the filler cap.