Bathtub Tom makes a good point. The other side of this is I have known a fill mark be set too high. Assuming we are talking about a transparent fill reservior, why not see if the lower level stays put? If it does - leave it at the lower level.

The other points are rather less likely, a cold not hot leak (usually the other way round for cracks in castings etc) and just perhaps, a stiff sealing ring on the pressure cap that softens and makes a better seal when hot. New caps are cheap so try one.