Shower sewer odor.

[Scrapper]

Kinda looks like they work the same way as dirt traps on heavy equipment diesel air cleaners.

[esmoglo]

Took it out & cleaned it up, there was some of my kids long hair inside keeping it opened but it doesn't take much though.

[Dwilcox]

Looks to me like you have plenty of room, I would install a p-trap. But that's me, I seen lots of those fail do to hair, tooth pick, tooth paste, Bobby pin,etc. I am a plumber and I feel water seal trap is the best. If it smells, add water, done.