Obviously take the car back to your mechanic and have them check out the new radiator and evaluate the source of the leak. If it did go bad it should be replaced under the repair warantee. Is your car a turbo? And if so is it water cooled? Could be a very slow leak when the engine is hot that evaporates and leaves very few signs. Check all the hose clamps and around the water pump. The expansion bottles develope cracks over time and can start leaking. After running the car let it sit for 30 minutes and then slowly loosen the tank cap. There should still be pressure in the system.