It was intended to be a clean out or a drain for some fixture-- difficult to know which. I do not see that a cleanout is required at that point. In any case, the hole should be tightly closed with the proper fitting.
That DWV system could not have passed the leak test with that hole open.

WOW, talk about shoddy work.

Upside-down santees and an inaccessible PEX "T".

I'd be call a plumbing inspector.

"Upside-down santees and an inaccessible PEX "T". "
Nothing wrong with either of those. The TEE in question is on a vent, no waste flows through it. No code violation to have concealed PEX fittings that are permanently installed.

"Nothing wrong with either of those."

And the open vent, is to be expected?

Pride in workmanship.

You two don't think the photo is from "over a year ago?" and was of work-in-progress?

Do you also both think the drywall guys missed a wall?

lol

Obviously the photo was made while the house was being built. And later was drywalled.
Is there now a fixture at that location? The cold water supply to the left of the opening (and to the right of the tub) looks to be typical for a toilet. The toilet flange may be just out of the photo.
It is not unusual for the drywall people to jump the gun and cover over unfinished plumbing or electrical work and that would seem to be a distinct possibility here.

Yes, bus driver, this is the vent behind the toilet. This photo is obviously mid construction. I have photos of all the plumbing, structure wiring throughout the house.

They are building the same floor plan a couple streets away so I was able to snap a shot of the same bathroom, same layout (on the right) and that opening in the vent isn't there. So I thought it might have been drywalled over without being sealed. You cannot get to it from behind the toilet. Something in this wall is not venting correctly so this would be the first place I would start.

I have a service visit scheduled next week so hopefully the plumber can figure out what is going on.

Obviously the toilet can be removed and the wall opened to inspect that part of the DWV system. It can later be patched so that it is not possible to see that the patch was made.
You do not mention the type of sewage disposal at your premises. City sewers often present little or no odor at the vents of houses. Septic tanks typically have incredibly foul odors at the vents since they have no other vents associated with the system.
If the plumber failed to close that opening, the repair in it's entirety should be on his nickel. Again, there is no way that the required leak test of the DWV system could have been done with that fitting open.

Hi, What a difference the second picture made. I looked at the first picture and thought maybe a wall hung toilet.
Woodbutcher

That fitting is too small for a toilet outlet.

"That fitting is too small for a toilet outlet."

You think he meant a full wall hung toilet and not a wall hung urinal? lol

A full wall hung toilet would have you sitting about three foot off the floor.

I suspect that the plumber did intend that for a cleanout. Nothing prohibits cleanouts even if none is required. The cleanout would have projected from the drywall or would have been flush with the wall with a removable cover plate. No demolition would be necessary to access it.

So to follow-up, that hole was left open. I'm so glad to just get rid of that smell! The builder sent a plumber and they will fix the wall.