During the smoke test, if you suspect a "Studor Vent" problem, unscrew it and duct tape the opening well to seal it. If the problem goes away the vent is bad. They're cheap and easy so I'd just replace them all with new ones as a starting point before doing anything else. Another "usual suspect" is the washer drain- I've seen many of those plumbed in without a trap. Usually not a problem but sometimes they are and it's not the right way regardless. Less likely but possible is that the vent pipe is leaking in the wall- have you drove nails or added a bracket for something which might have penetrated or cracked a pipe? You can check that an inaccessible trap is holding water by lowering a weighted cotton string into it from above- if it comes out dry the trap is bad. Do this test after letting the trap/drain go unused for a couple days if possible to give a bad trap time to lose it's water. And wherever the smoke is, there's your leak. It may be easier to pinpoint by closing off the room making it dark, then taking a small narrow-beam flashlight to look around. That will highlight the contrast between the smoke and the other air in the room.
The fix you need doesn't seem big from here but you've got to find it first.
I checked the washer and there is water in a trap. Studor vents: one of two in the house (the one in the bathroom in question) was replaced already when trying to trouble shoot it before. I have not hung anything on the walls in question, so not likely I pierced a vent line. Possibly when building but as Phil said, not likely.
Seems like I will have to schedule another smoke test since (once again) I cannot find anything obvious. I just want my house not to stink!