I'm willing to accept that there is possibly some kind of backdraft from the gas furnace and/or water heater. I find that the smell is usually only later in the evening or at night. My guess is the chimney is cool enough at this point that when the furnace turns on, the exhaust may backdraft for a short while until the chimney heats up enough to pull the exhaust upwards through normal convection. I leave a window cracked where the furnace is thinking it might need extra combustion air but i'm not sure if this is making the problem worse by creating too much air pressure and pulling the air upstairs to the living space.
I have a new CO detector on the ceiling at the basement stairs which is 10 feet from the furnace so I assumed this would beep if this backdraft was occurring. I've found cases where people with gas fireplaces say they light a newspaper and hold it in the fireplace to preheat the chimney but obviously that is a little impractical for my needs. Still a little stuck here.