We have a light "off" odor in our bathroom. It could be sewer, but because it is not strong it is hard to tell. The smell could be something else. Also because it is a faint smell, it is very difficult to identify the source of the smell beyond the room.

Here is some back history.

We recently purchased a home and had extensive remodeling done. Nothing of concern was identified in the area during the remodel. In the bathroom, the only thing that was not changed and brought up to current code was the bathtub and the tile above the tub (shower walls). During the remodel, we did not identify any previous water stains and there was nothing to indicate water damage had been present anywhere in the bathroom. We have not opened up the walls, so I cannot comment on anything there. This bathroom is on the second floor and there is no damage to the ceiling in the room below. The "wet wall" of the bathroom on the second floor lies in the middle of the ceiling for the room below.

When you put your nose to the bathtub drain, it smells like shampoo. No discernible smell on the shower walls.

We have new tile floors and a new toilet. The grout around the toilet on one side appears darker and looks wet compared to the other grout. The feel of the grout is that it is dry. When you put your nose to the darker grout, it doesn't smell different than the other grout. The grout on the other side of the wall is normal looking.

There is no apparent "gurgling" when you drain the tub.

Any ideas on where to investigate next?

Second floor so maybe check the vent pipe outside which should be on the roof for blockage. I am not an expert of any kind just a little experience.

Can you check the attic if there is one? Dead mice have a unique smell, as does their excrement. Any animals that might get in.

A remote possibility that the vent wasn't reconnected in the attic before going out the roof, so sewer gasses are drifting into the attic. A remote possibility that the wax ring under the toilet wasn't seated properly and you have just the tiniest of leak that dries up before it gets to the ceiling below.