The one on the right is larger diameter, but the one on the left has the cleanout, so what’s this photo showing?
Looks like new construction with that hanging PEX. Maybe radon system not fully installed yet? Hard to see where it goes in that photo.
Sounds plausible to me. It’s not connected to the waste pipe that comes from that side.
They’re both serving smaller diameter drain pipes. A radon vent pipe wouldn’t branch into smaller pipes.
Kenton, I have to go with the radon vent…corner install of this pipe is pretty common. I look at your pic and do not see the smaller pipe connecting…I see it running parallel.
Is the house on a septic system? If so, it may be a grey water line.
That pipe on the right appears to turn horizontal and vent to the exterior through a rim joist. There’s a smaller- like 2 1/2"- pipe right next to it.
I’m used to seeing radon risers outside, but expected to see a fan in this situation.
Passive Radon and a fan is only installed on the outside** if** it is needed.