Could be over flow leak from the radiator cap. If there is a definate radiator leak, then the coolant level will be affected, where you would be topping up regularly.

Keep a regular check on the coolant level from only a cold state. If level is not affected, then you have no leak to speak of. If the level is affected then you have a leak to locate. If the radiator is not the source of the leak, you need to check all the pipes connections, consider the water pump, these can be difficult to identify, in particular if the it has a self sealing leaking, which only tends to leak during a warming up and cooling down conditions, any period outside that, the leak self seals.