In that case, it sounds like it's normal. Talking only about an engine that's at operating temperature, after you shut the engine off, The coolant actually gets warmer/hotter for a while. When the engine's running, the coolant absorbs heat from the engine and then sheds that heat in the radiator. Since the coolant's no longer flowing after the engine's shut off, it will still absorb heat from the engine, but it can't get rid of that heat. The cylinders and especially the heads (around the combustion chambers) are hotter than the coolant is so the coolant will still absorb the heat until everything is at the same temperature. Then, over time, the heat will transfer from everything to the air and it will all cool down. Since the coolant still gets hotter after the engine's shut off, it will build more pressure in the cooling system. Often this will be enough to make the radiator cap open and let some coolant into the overflow. As the coolant cools off, it will draw back into the engine/radiator from the overflow. I hope this helps you understand what's happening.