That's a strange one Bob - I thought it would leave the tank full just after a run...then as it cools down it'll sook some coolant back out of the expansion tank?
If it's taking coolant out of the expansion when hot then there could be an issue?
When my rad was leaking it was sucking more coolant into the system in out of the expansion tank (not sure if after or during the run) and then it would eventually start sucking air in and giving the symptoms Olie was describing.