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.

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Add half a bottle of water and good to go.