Probably the engine head gasket has developed a small leak. A little bit of coolant might have managed to escape from the coolant channel and got between the engine block and engine head. May need a new head gasket, and need to check if the head surface to the engine block is within the specification before putting in the new head gasket. Need to do a pressure test to verify the problem. Also check and see if you need to add coolant to the radiator from time to time.
This problem can last rather long time before it becomes obvious that you have to take care of it.
