My car overheated while the engine was off and radio on. What's the problem?

My car had overheated last week so I took it in to get fixed. They replaced the thermostat because that was the only thing they thought was wrong. Everything has been fine since, except today when I went out. I drove the car about five miles then parked, turned off the engine and turned on the radio. I was sitting there for about ten minutes when I noticed steam coming from the hood. It had overheated. Any ideas what the problem is? I was told that it could be the thermostat is blocked and the blockage needs to be cleared. I just want to know before I take it back in tomorrow. I noticed that after I let the car cool there was moisture around the cap.