Coolant Leak when using heater????

[TNT79Bronco]

Yet another "Coolant Leak" question. I did some research, but nothing specific to the issue I am facing. It seems that when I drive around on non cold days (meaning I don't use the heater), my coolant level seems to stay constant and at the top of the reservoir.

However, when I use the heater, I can smell a faint whiff of coolant and after driving around a while (a couple of hours), I can see that the level in the coolant tank has dropped about 1/4"-1/2".

I am thinking heater core (which I replaced not too long ago, when there was a bad coolant smell, and I could see coolant at the bottom of the heater core housing), must circulate hot water when I activate the heater, thus maybe another leak.

I just assumed that the heater core always circulated hot water, no matter what the temp setting, but maybe I am wrong? What could be an explanation for this unique coolant loss?