Question about hot water boiler, baseboard always hot

So I have a very old hot water boiler on a two story house with no basement. Lately its been rather warm out so there is no call for heat from the thermostat upstairs but some of my baseboard is still hot. Thus making a few rooms very warm. In the boiler room the pump for that zone is cold so I know its not running. I follow the supply pipe and it seems to be hot but the return is cold. Is this normal? Would the water move with the heat on one end explaining the heat upstairs?

 

Failed zone valves or failed check valves can allow convection to occur in systems, but without a better picture of how your zones & pumps are configured it's hard to get more specific than that. With very low convection flow only part of the baseboard loop will be hot, and the return water can be fairly cool.

A picture of the near-boiler plumbing and the pump/zone plumbing might be useful here.