Have a customer, owner of a strip mall. One of the units came down with a bad odor, smells like something died but they cant figure out where the smell is coming from. Has persisted for over a month he says. I had my dog check it out. I thought he zeroed in on one wall but then he lost interest. I pushed up a ceiling panel. Clean up there, no smell. There is an air conditioning vent near the wall my dog hit on. The owner said he had the ventilation system cleaned and they didnt find anything. Thoughts or ideas? I suggested he seal the vent with plastic to see if the smell goes away.

If you think it might be in one section of wall were your dog found something , do this -

Remove the baseboard molding near the floor carefully as not to damage it. Then punch a hole with a screwdriver in each section of studs , making the holes where the baseboard was to be able to cover up when done. Smell each hole. You will know quickly as the smell will be obvious in one hole. Carefully cut a slot smaller than the baseboard and if the DOA is small enough you may be able to slip it out and replacing the baseboard will cover any exploratory surgery.

Excellent idea, thank you!

More often than not, in a commercial location. It is a floor drain or sink that the P trap has gone dry allowing sewer gas to enter the area.

Lingered for month? Consider climatic conditions. Most decomp is over and done within a month. Winter can linger due to cool conditions. Summer gets intense quick but over quickly as it dessicates. Lingering odors I look for moisture and cool conditions that or mechanical such as mold or mildew.

I have good luck using https://www.wildlifecontrolsupplies.com/animal/NWS53070.html. I have four of them.

Got some intelligence in the room tonight,,,,,,,...........