... The drain vent is going out the roof and it seems to have very good outflow (smoker thingy used). The smell does appear to come from the drain vent tho', and we had someone in today (I couldn't wait any longer, I needed to do laundry) and resealed the cap. He wants us to wait a few days, air it out, and see if the smell resurfaces. If it does, it's time to break into the wall.
He did do testing and the walls are dry.