I have a conference room with a sewage smell that comes and goes? How do I get rid of it?

I camerad the vent pipes in the floor and found nothing. I have checked the roof vent pipes in the area and they are clear. I have even checked in the crawl space in the area above the room, but still I can't find the source. Any suggestions?

Okay, we know it is coming from a drain line, or a vent line. That is the only source of sewer gas.
#1. Check for a floor drain - perhaps covered with carpet. Take a good magnet across floor, if you contact something that makes magnet attract, you may have located a floor drain that was carpeted over at some time. When the trap dries out in the floor drain, sewer gas can escape. If not under the carpet, check adjacent other rooms for a floor drain. If you find one-pour 1/2 gallon water into drain to fill up the trap.
#2. Sometimes heating and ac people will run condensate drain from the a/c coil into a sewer line. This is against code, but in places where there is no area to run drain they do this.
They configure a trap in this drain to keep gases out, however this drain can also have water evaporate out, thus sewer gas comes up and out of unit. These drains are usually 3/4" pipe and come out of furnace where a/c coil is located.
The other thing you might have is an in line pro-vent. Some plumbers put these in at a kitchen sink instead of running a vent pipe up and out. These are a small 2-3" diameter fitting which may be located behind the drywall in the wall above the sink. or under the sink. The only other place would be a disposal or dishwasher that the drain hose has lost its trap feature on.
Perhaps your waste vents that go through the roof are either close to the roof vent ridge or other roof vents- when the wind blows it might suck some of this smell out of the vents and it then drifts back through some of the roof vents-then dissaptes down through the ceiling into the conference room.
See if the smell is mainly when the wind is blowing.
I would say you'll find something following these tips.
Good Luck

Just a wild shot but I ve had to do it from time to time.. If you happen to be in an office that has a cement floor.... There may be a drain covered over by carpet? Ours was and we had to cut around it and have it exposed because the trap had dried out ( water in the trap) All we had to do is once in awlie ( we did it on a monthly basis ) is pour some water down it.. Just a wild guess GL

don't worry. you nasal passages will eventually get used to the smell after a week or so. Seriously- you won't beable to smell it again if you are in this same environment for long periods of time.

get an ozone air purifier, they often use them in toilets and offices to get rid of smells.