A bedroom has a ordor similer to sewer smell but very faint? What might it be

I had a similar problem that was ongoing for over a year, a bad fish smell, after several attempts to discover the problem, I noticed that the smell only appeared after I had turned the light on, I took the shade off and discoverec the white plastic light fitting had turned brown and was giving off the smell.

Might not be that in your case, but worth having a look.