In the case of my leaking heater core, the symptoms were coolant smell in the cabin and a haze on the inside of the windows, especially when in defrost mode. My carpets never got wet. The heater core is in a "well" inside the HVAC box, and it seems to me that there would need to be quite a bit of coolant in the well before it would leak out onto the floor of the car. When I took my leaking core out, there was just trace amounts of coolant in the well, that could be wiped out with a rag. Here is a picture of my leaking core - enough to be noticeable when driving the car, but not dumping coolant out.
