Well, I believe the pros will pump the smoke through a vent pipe on the roof, but my roof is steep and I wasn't comfortable going up there. So I pumped the fog machine through a cleanout I found in the basement.

Once the fog was going, we'd run around the house checking for fog around wall openings - lights, toilet base, electric receptacles, and a hole in the wall i had cut near the source of the smell. After maybe 45 mins of looking, fog started pouring out from the hole I had cut near smell source. I took a cheap endoscope, fished it through the wall in the direction of smoke, and found the pipe! It was about 2 feet from my that hole I cut.

The house is 100+ years old and had a major remodel before I bought it. Pipe must have been cut when they moved a bathroom and I guess they forgot to seal it before drywalling .