It is possible that some water is getting past the valves, depending on the power of the circulating pump, and the state of the valves.
Also, there will always be some convected heat as the rad is on a circuit that has hot water flowing past it. It will depend on the length of pipe that goes from the circulating ring of hot water and the radiator. if it is short, then it is possible for the radiator to heat up.
You have to ask yourself, if it is enough of a problem to warrant the hassle of replacing the valves (draining down the system etc.). If the other radiators have enough heat, then maybe it's not.