It may be something with your heater hoses or core but if it had a leak you would definitely be losing water.

Have you recently spilled coolant inside your engine bay becasue that smell will stick around for a while. I had a heater hose leaking and when I replaced it, I still smelled coolant for a week or two.

If you think it has something to do with the heater system, just reach up under your dash near the center and see if you can feel any dampness or smell anything there.