I had the same thing happen once,
When I had checked to make sure the radiator was full doing some maintenance before the flight, I unknowingly allowed the radiator cap gasket to fold over itself slightly when I closed the cap, then it wasn't sealing properly.
It boiled coolant all over the place after takeoff.
I landed quickly right after takeoff and could hear it sucking air in through the leaking radiator cap gasket as it cooled down. If I wouldn't have checked it out right away, I probably would have never found out where it was leaking, since when I removed the cap it straightened itself out.
