Check the radiator cap is sealing properly if not, it can puke out coolant into the overflow when hot and not draw it back in when it cools.
Tomorrow I will check from the radiator cap if the radiator is full.... And yes that it could be.
@marcom started
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Check the radiator cap is sealing properly if not, it can puke out coolant into the overflow when hot and not draw it back in when it cools.