Smelling antifreeze when I run heat

I filled up the antifreeze and it dropped maybe one-half inch. There is NO residue on the windshield or windows, and no leaking antifreeze through the floor heating vents. And the thermostat appears to be working fine. But the engine smells like burning antifreeze if I use the heat but there is no overheating or leakage of antifreeze…

I’m really confused as to what it could be.