first of all, check for leaks in the pipes and the radiator itself, sometimes they only leak when the engine is running.. since the car's cooling system is a pressurized cooling system, pressure builds up, pipes or something else (leak in the radiator) can't handle the pressure and leaks steam / water slowly. You check this by running the engine till its warm / at operating temperature. Open the bonnet and look for any coolant leak.. check the radiator closely using a torch. check the pipes.. look for steam or coolant.. goto a mechanic and make him look into this. Don't buy what he says.. first make sure that you don't have a leak ANYWHERE. Worst thing, you could have a slight leakage in the head-gasket. Come back here when you're done checking. Good luck.