Mysterious coolant

Most likely is the heater core, as steam. But on a cold night you should see it fogging the windshield (my 69 Dart did). It might also be a pin-hole leak in a heater hose, like my sister had in her 65 Mustang on a trip. It kept boiling over every 50 miles on a long trip. One shop said "must be head gasket", so we kept driving. A second shop found the leak and fixed it for $5. It didn't start until the pressure was up and it didn't even leave a noticeable wet spot, but did lose significant water.

Whatever you do, don't approach it like a coworker did with his late 90's GM truck. He asked my opinion on a 1 gal/wk coolant loss. I asked if he had smelled coolant in the cabin last winter, he said yes and that the windshield had been fogging, so I told him must be the heater core. Instead, he listened to a dealer mechanic and started shopping for another LS engine, then ended up buying another used GM truck.