Do you smell the sweet odor of antifreeze when you have the heater on in the car? If you do, then the leak is in the heater core.

Since removal of the heater core is a PITA on most cars, using a stop leak product is worth a try. I've had cases where Bars Leaks will work, but it makes your coolant look brown and ugly.

As mentioned, another old standby is to add ground pepper to the radiator, same as you use in the kitchen. Believe it or not, it can build up around a small leak and seal it.