check the hoses, follow the hose from the thermostat housing and see if there is a loose clamp. Could be something that simple. If the engine is running smoothe, I wouldn't think it has any major problems. With it running you should be able to feel the thermostat housing or the power head for that matter to see how hot it is. they only run at about 140. With no alarms you should be safe running it a short ways.
We had a rough winter this year. If it was stored in the tilted position or laying on the floor it could have froze and split the water jacket. I have been flooded with calls from guys in the past 2 weeks that never had a issue with there engines freezing in the past but this year they froze. They all claim they did nothing different.