Smell in stairwell from basement

We recently moved into a townhouse that has a finished basement, a main and second floor. When we open the door to our deck on the main floor, we smell a rancid, gassy like smell in the stairwell to the basement. The smell doesn’t come into the main floor and doesn’t go into the basement area. With the door to the deck closed, we don’t notice the smell. We also don’t notice the smell with a second floor window open and the door to the deck closed.

The basement and stairwell had carpet which we thought was the problem but it was removed about two months ago and the smell remains.

At the bottom of the stairwell to the right is a furnace room and under the stairs is a storage area. Neither of these areas has the smell. We’ve had the gas water heater and furnace replaced. Both installation people didn’t know what the smell was.

At the bottom of the stairwell is a full bathroom with a whirlpool tub which we’ve never used. The bathroom appears to have been added at a later date, the house was built in 1985. Nothing in the bathroom has an odor like the smell. We’ve smelled the drains in the sink and tub, the openings in the whirlpool tub and the gap between the tub and floor.

As an experiment, we closed the bathroom door and put a towel on the floor. No smell with the door to the deck open.

We then opened the bathroom door and plugged in the drains in the sink and tub. We noticed the smell.

We then put the towel in front of the gap between the tub and floor. We noticed the smell in other areas of the basement that never had the smell.

There is an access hole for the tub in the furnace room which I checked. There was no smell evident when I opened it.

I cut a new access hole in a closet on the other side of the tub and didn’t notice any smell. The concrete floor had been cut to install the drain then the hole refilled. I don’t know much about plumbing but there isn’t a vent pipe directly attached to the drain pipes that I can see.

We have a dropped ceiling so I also checked that above the bathroom, nothing unusal. We have a humidifier in the basement that doesn’t collect an excessive amount of water, so I don’t think it’s a moisture issue.

I haven’t checked to see if the smell is noticeable when any other main floor windows are open.

Does anybody have any ideas of what this smell could be and why we only smell it when the door to the deck is open??

Any idea of how to get rid of it??

Thanks for your help.

 

I wanted to add that we've used the shower, sink and toilet in the basement bathroom regularly just not the whirlpool tub.

 

Dead animal in the wall, ceiling, floor, duct?

 

Another possibility is the bathroom plumbing vent pipe was dead-ended in one of the stairwell walls. Opening the outside door creates a negative pressure, sucking some sewer fumes out of the pipe and into the wall cavity, where they work their way out through the tiniest of openings.