An important law of physics tell us that the pressure of a fixed volume or gas rises as the temperature rises and falls as the temperature falls, such that pressure x volume equals a constant x temperature. So, a cooling system at ambient temperature that's opened will be set to atmospheric pressure when you next drive the car. The cooling system will/should return to that same ambient pressure after cooling down to the previous ambient temperature. Any significant overpressure thereafter means there's more permanent gas in the system than before. Translation, there's exhaust gas pressurising the coolaing system (even a bromothymolblue test notwithstanding).
TC
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