I hate leaks like this. Go to the local auto parts store and rent / loan the coolant pressure tester kit. It comes with an adapter for your radiator. MAKE SURE to not exceed the pressure listed on the radiator cap. Once your pressurize the system, engine off, you can find leaks pretty quick. Easy to hear coolant leaking and being able to see the pressure gauge drop.
If the leak is slower than that - buy a little bottle of coolant UV dye and some glasses and cheap UV flashlight. Add some dye to the radiator and run the car normally for a few days. The pull it in the garage, turn off the lights, break out the glasses and the UV light. You'll be able to spot the leak very quickly.