A few years back my dentist noticed a smell in his office and traced it down to a floor drain by the water heater in a closet. He called building maintenance and the poor guy thought something had died in the drain and stuck a shop vac nozzle down in there. Well that immediately made things much worse. Fortunately i was in for an appointment and told my dentist to have the maintenance guy slowly pour a bucket of water down the drain…Problem solved…It’s almost always a dry trap somewhere Junior…
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Yes, possibly that’s what they’ll find. It’s been quite a few years since the renovation. Smells only came on this year.
Next time add some mineral oil it will greatly reduce water evaporation from the trap. Vegetable oil work well too. I’ll never understand why some commercial buildings go cheap and don’t install trap primers.
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i believe I mentioned that to him but for immediate use with an office full of gagging patients the water got the job done…
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When the contractors renovated the building, they removed all the blood-stained carpets, you’d think they’d be smart enough not to cover a floor drain! 
Dead mouse in the HVAC system 
They scanned the ducts and pipes with a camera apparently. It’s a sewer smell.
I can never video the entire duct system. Once I hit the main plenum it’s over.
Mice don’t really smell like sewer though. I live on an acreage, get to observe the different smells quite often! 
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If it was a hidden floor drain, would it have taken 20 years to dry out and start stinking?
Maybe they had something set on top of it for a long time?
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Good point, will find out if they removed anything, be a good way to narrow down the spot possibly, lol. They did go paperless, perhaps removed some old filing cabinets or something!
Did anyone confirm it was not a gas leak. I have had homeowners get the smell mixed up. Maybe some should time the meter.
It’s sewer, plumbing/heating guys been there just haven’t figured out why yet I guess. They’re going to move things around more, look for other floor drains, or something hidden, etc…
I’ve seen this happen on a house when the condensate line “improperly” was routed to a vent stack in the attic.
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Thanks everyone! Once out of quarantine, they’ll hopefully get it figured out!
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Keep us posted, Junior…I’m curious. 
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Will do, Larry!
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Check floor drains, the traps dry out and sewer gas enters the building. Put water and a cup of vegetable oil in the drain the oil helps stop evaporation.
Likely several ongoing issues.
When a trap is functional the trap empededs odors from entering that space.
From what I have read, seeing this anomaly appears to be within the building, I suspect commercial building, air is not circulating in the building as intended. Doors to main offices must be sealed.
“it gets much worse when the furnace is running” leads me to believe stack effect might be one consideration. Buoyancy occurs due to a difference in indoor-to-outdoor air density resulting from temperature and moisture differences.
Doors.IBC 716.5.3.1 Smoke and Draft Control
Fire door assemblies shall also meet the requirements for a smoke and
draft control door assembly tested in accordance with UL 1784.