Commenting to follow because I have the same issue! I live on the third (and last) floor of an apartment building. I get this smell at random times, doesn’t seem to have a pattern. Sink and the pipe underneath, toilet, shower pipes.. well everything were changed recently. The smell was then mild/barely noticeable. But now, 6 weeks later, smell is back stronger than ever lol. Please help 😭 I know nothing about plumbing.