Just a thought but mine was the Auxiliary blower motor (or something), I could smell coolant when my heater was on but then the car also started loosing coolant at an alarming rate, the leak was from the left side of the bulk head and you could hear it sizzling on the hot exhaust. Turned out it was the blower motor in the engine bay (the one on the left of the engine bay by the bulk head, next to the fuse box - sorry don't know what its called, some of them have a cover on but mine doesn't). The inside of the blower is made of plastic and it had just broken away. Anyway while it was on its way out you could smell coolant but there was no noticeable leak. Maybe someone knows what I'm on about and can add a name to my useless description!
The good news is it was cheap to sort out which is rare! Are you loosing any coolant?