It could be the vent for that bath. You need a plumber. If it is the trap, is there a wall that you can go through to access it without removing the tub - a closet wall or wall on the other side of the tub - in a bedroom or hall?

Thanks to both of you for responding!! Unfortunately the tub is on a wall between our unit and our neighbors. The tub is rarely used but we have to run water constantly to keep the trap full and even then we can still smell the sewer gases.

There is a possibility that it's not the tub. We experienced a sewer gas smell in our house shortly after we moved into it, especially in the mornings. I ran water in all the sinks, tubs, and showers, just in case the traps had dried up between the previous owner's and our possession. The smell kept coming back. We finally pinpointed the problem: A cracked joint in the plumbing ventilation system in the attic. We suspect our inspector stepped on it inadvertently during his inspection since the space isn't finished and the line was partially covered by insulation. It was a fairly simple and inexpensive repair, and the smell vanished.

Maybe there is no P-trap or the tub is improperly vented which creates a vacuum which pulls the water out of the trap as the water is being drained.

There is tools to see if you have a trap or not, I would call a plumber, most likely they have a portable camera tool, or they have other way of finding that out. If there is a P-trap, you might have a vent issue.

Good luck.

Thank you both for the info.