My hire car said this to me yesterday:
Check coolant level. So I stopped near the Czech border and, er, checked it. Modern cars have become hideously complicated and the days when the cap was on the radiator are long over but I found something that looked right. It seemed fine and the message vanished.
This morning same thing. The car had been parked in the same place on a slope with the right hand side higher. A stroke of genius came to me. The slope was just enough that the water would move away from the sensor.
Add half a bottle of water and good to go.