Had a similar problem last week. It turned out to be a cracked inlet to the radiator where the hose enters the radiator from the water pump. Had to get the tanks replaced with metal bands inside. It was causing the temp to rise just slightly and putting coolant around the edges of the hose, where it enters the radiator. It was not noticeable either. It was leaking underneath hose clamp, little by little. Check hoses and do coolant flush. Then, you should find the leak hopefully.