Well theres a lot more pressure when its hot - and it gets super hot immediately after a drive as there is all the heatsoak from the engine but no coolant circulation. This is why you often hear coolant boiling in a vehicle after it has been shut down even if it was fine while the engine was running.
If you are having trouble locating the leak when its hot then put a pressure tester on the radiator when cold and bump up the pressure in the system until it starts leaking. That way you can safely get under it and search for the leaky pipe with no risk of getting burned.
Start with the usual things though like thermostat and making sure that your radiator cap is fine (as a faulty one can let the pressure get very high and spring leaks all over the place)
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