Big-Spend-2915 t1_ixlx09p wrote on November 24, 2022 at 1:02 PM

Your HVAC system could be putting that smell in there. Check for that also. It can draw the gases into the ducting and blow it back out.
Had that in one house, one bathroom smelled, off and on. Tried several different things. Even did a smoke test and nothing. Ended up finding a vent pipe going into a cold air return that had a joint wasn't glued by the plumber. Only way I found it was by removing the cold air covering and pulling on the pipes.
Another is possible the toilet seal has failed. Next, look for anything that was added recently. A screw/nail could have went through a pipe.

otr_otr_otr OP t1_ixm0mtj wrote on November 24, 2022 at 1:38 PM

Weird, I could have sworn that I had responded here already. Anyway:

Could you speak a bit more about the seal? If I assume that the underfloor siphon and ventilation work, then the portion of the pipe that could be affected by a leak in the seal would only contain a limited amount of smelly air, right?

This toilet is stepped on regularly, for example to turn off the gas outside. I also did install a new toilet seat, which doesn't involve drilling, but a fair bit of shaking and movement - could these things have unsettled a seal?