Septic-like smell from wall

[bbender716]

About 3 days ago, I began smelling an intermittent septic-like smell coming from the pictured wall.

General layout info:
We live in a one story ranch with a partial finished basement. The furnace/main mechanical room is near but not directly below this bathroom. The "smelly" side of the wall is the wall above the stairwell.

The strongest smell was on the side of the wall that is above the basement stairs and NOT the bathroom side.

We had a plumber evaluate and his guess was the toilet seal/flange. He replaced the wax ring, toilet flange, and recaulked. This was yesterday afternoon and unfortunately, we're still getting that smell in the same place.

Weirdly enough, the smell goes way randomly but comes back with a vengeance.

I climbed into the attic, which annoyingly has blow in insulation everywhere. I tried to dig down to approximately where that wall is but didn't smell anything strong except normal attic smells. Tried stirring up the insulation to see if anything dead was there or at least especially smelling but didn't notice anything.

Also confirmed that besides the exhaust fan in the bathroom that no other venting was coming up through that wall and exiting the roof.

I'm kind of lost on what to investigate next or what other possible explanations could be. How can I determine if there is pipe in the wall? Is there a way to avoid cutting a huge chunk of drywall out above the stairwell to determine the cause?

Thank you all in advance!

http://imgur.com/gallery/0DN1B8a

 

[Reach4]

Weirdly enough, the smell goes way randomly but comes back with a vengeance.

Note the wind direction and intensity during smell/no-smell.

 

[Sponsor]

 

[bbender716]

Note the wind direction and intensity during smell/no-smell.

Will do. Thoughts on what that'd indicate?

 

[Reach4]

Will do. Thoughts on what that'd indicate?

Not exactly. It was a thought as to what the random nature might be, but I don't know that that would point toward a cure.

There is a test called a peppermint test. Maybe read up on that.

 

[Tuttles Revenge]

sniff tests are subjective, I'm virtually immune to all but the strongest sewer smells. Hire someone to do a smoke test if it isn't solved by resetting the toilet very carefully.

 

[Lucy20]

Will do. Thoughts on what that'd indicate?

Did you ever figure out what this was? We are experiencing something similar!