Coolant is at the borderline to set the sensor
Coolant level is just below the sensor borderline. As the engine heats up, the coolant expands, turning off a low coolant warning light. When it cools the coolant contracts allowing it to drop below the required threshold and illuminates the light again.
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Coolant System Leak
A leak in the cooling system will diminish the level of coolant. If it is a very small leak, it may take a while before a problem becomes evident.
Engine coolant is usually a combination of water and coolant. The water can evaporate, leading to a reduction of coolant. The coolant will have a higher gravity, or density, if some of the water has been evaporated.