I think this comment is on the right track. Methane is lighter than air. A break or fault in the vent stack in the floor below would tend to rise.

I remember staying in a hotel in Oaxaca Mexico. Nice place but the floor drains lacked traps. I think they put a trap under the toilet but however they tied in the floor drains it bypassed that trap. The shower area always stank just a little bit. So you could be looking at a design flaw.

Or, there can be a penetration in the vent stack-- Crack, loose joint, nail hole.

I know of two methods to assess. One is a smoke test. Basically they open a clean out down low and pump smoke in it. Wherever the smoke comes out needs to be repaired

The other is by using a boroscope inspection camera. You can drop it down from the roof or anywhere there's a clean out and just inspect all the joints.

So I would ask around for plumbers that are familiar with these techniques. My first choice would probably be the smoke test.