I found an unusual plumbing configuration under a sink today.
where you might typically find a vent pie or at least an aav (aka studor vent) instead there was a clean-out cap. Drainage seemed fine during several minutes of running water and running the dishwasher also. The floor was a concrete slab so I could not see the pipes below the floor.
I think this should have a vent. What do you think?
I agree there should be some type of vent, with two traps. I’m surprised there wasn’t/isn’t a foul odor from the waste disposer
In my area, I actually see this all the time, and nearly all homes are on slab. I called it out a couple times, but its so common, I dont anymore. Never had an issue with anything draining.
It likely vents at the nearest wall.
I see it all the time too, mainly because my own kitchen sink doesn’t have a vent. Yet it drains perfectly fine with no issues or gurgling. 
So this was a little uncommon, but the disposal had a trap of its own, so it was protected from the drain pipe to the sewer. Hard to see that second trap in this picture.
There is… the other sink bowl!
Test: Fill BOTH bowls with water, and drain them together. Guess what happens?
That’s usually when the issue will present itself of course. Funny thing is that darn near no one fills their kitchen sinks to do dishes anymore.
This is a common configuration in Florida. Usually for a kitchen peninsula or island sink, there is likely a vent pipe in the nearest adjacent wall.
2020 Florida Building Code, Chapter 21 Vents
There are a lot of requirements for island sink drains. The biggest clue should be the size of the drain plumbing. There is a table in this chapter that show the distance a vent can be away from the drain trap based on the size of the drain plumbing.
As for no visible drain vent, I rarely see the vent unless it is an island then I typically see a AAV.
What I do not see that is required, no clean-out plug or clean out provisions on both drain lines.
Morning, Bert. Hope this post finds you well.
I might be wrong but I refer to the plumbing fitting atop the vertical DWV/drain line, a Cleanout Adapter with Plug.
There is no physical way for a homeowner to clean out the traps without damaging the assembly. No trap plugs and no clean-out provision.
Here in GA, we are governed by IPC 2018. I have been unable to locate the requirement for clean-outs at the locations you have pictured. In the field, they are also not present. There are other clean-out requirements down-stream.
That’s one strange drain line set up, at least IMO, and I can see the reasoning in calling out the lack of a proper vent if there is no access below or no AAV installed. I always fill both sinks at least half way full, then drain them at the same time I drain the DW and run the disposal, and if all drained without any visible issues, I wouldn’t write it up.
