Mystery odor in living room. Please help!

A couple months ago we noticed a foul odor in our living room.Some characteristics:

• smells like stinky cheese or stinky cardboard
• more noticeable in the evening, although now it’s pretty muchan all-day thing
• the odor kind of lingers and moves around a specific corner ofour living room

What our apartment management has done so far:

• cut several large holes in the drywall and sniffed inside theholes. Nothing. The inside of that wall doesn’t smell.

• poked holes in the ceiling and sniffed the holes. No smells.

• checked the carpet, furniture, etc for rotting food or spilleddrinks. Nothing. Our furniture and carpet don’t smell.

• cleaned the air ducts

• testing the air for mold (negative)

It really stinks and we are growing extremely frustrated.Management ruled out electrical, plumbing, and dead animal. Wouldlove some input if anyone has experience with things like this. Idkwhat to do. Management probably thinks we are crazy. But there’sdefinitely a foul odor. One of the maintenance guys smelled it. Myguests smell it. It’s definitely there. We just don’t know what itis or how to go about finding the source.

Idk if this helps but here’s a picture of the wall.The smell is coming from that circled area.

We have a neighbor on the other side of that shared wall.Management said that wall is a “double wall.” I’m not surewhat that means. On the other side of the adjacent wall is mybathroom shower. No plumbing on those two wall, btw. Neither mynext door or my upstairs neighbors smell anything foul.