Radiator cap. There is nothing else that is supposed bleed pressure out of the cooling system. The situation you describe is very strange though. Normally when the car gets warm, the thermal expansion increases the pressure until the cap vents at 15psi or whatever it is set for, then as the engine cools the pressure drops back to atmospheric or below. Was it very cold overnight between your afternoon cruise and noticing the puddle and was it then warm? If so maybe nothing is wrong. What kind of radiator cap is that?
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