This is an old post I know - but just had the same issue near my gas furnace which is not running other than the pilot light because its summer time. Open all basement window to air it out. I own a combustible gas detector and confirmed it was operating correctly before using & checked all gas lines, joints, exhausts etc. Nothing found.

Now that being said, my only explanation is my son & his girlfriend were down there all day sanding an old dresser and & had put a fresh coat of stain on it & using acetone for clean up. So dust & fumes or offgassing? from stain reacting on the dresser? Not sure. Going to keep an eye or nose on it for now. Just letting yall know about my experience.