There could be a leak in one of the heater hoses or the core itself may be leaking. I would check those first. Another not so happy instance is that the head gasket is leaking, which could point to either coolant leaking around the gasket and down the block, or into oil passages and into the crankcase, which would be bad. If the car is running good otherwise I would check the simple things first, then delve into the more worrysome areas.
Does your heat seem to be working as it should be? Does the temperature of the coolant at the radiator/motor match the temperature you feel with the heat on full hot? If your engine is warming up as it should and your heat is subpar, then it most likely is a issue in or around the heater core.
Good luck figuring out your issue. I hope it turns out to be something simple.