Well that was an adventure. Turns out the person who built this building left a septic drain pipe uncapped and then built a wall over it. Like with the insulation and everything. We’ve been trying to track down the source of the smell, and when we finally pulled the plywood off that section of wall, the surge of smell told us right away that we were in the right place. Whew! Not my favorite way to spend a Sunday evening, but so happy to have found it and capped it!