Hi Guys,
Basement of my mom's house with galv and cast iron drain lines.
I can't figure what this pipe does.

It looks like it "should" be a vent
The bottom san Tee is separated from the top one as the nipple rusted completely
The water leak is very recent - couple days only
I can't figure where the water is coming from
The upper vert pipe has no obvious fixture in the house above that that would be a drain for
The horiz pipe coming out of the upper san tee runs to the front of the house and into a pipe that drops down to the house trap - I would assume that was a vent
The horiz pipe coming out of the bottom tee runs to the side of the house and disappears vertically at exterior wall
I was running water and a drain cleaning product thru a slow garbage disposer at some point in the day
at another point in the day my sister saw water running out of this pipe break
looking at all the pipes it make no sense that it could have been coming from disposal

I know it 's not much to go on but anyone have a thought on what's going on?

edit: we did have snow a week ago...couild be some melt sitting in pipe and the nipple just wore thru in the past day, but doesn't explain why it leaked enuf to just be damp and then when my sister looked she said it was "running" and then when I went down and looked later it was just wet with maybe a drop of water on the bottom of the cleanout



Edited 2 times.

My guess is the installer installed the clean out tee in the wrong direction and wrong fitting it should be a wye, the kitchen drain is clogged, the acid you dumped down the drain ate away the pipe or pipe dope which is common, cut out the tee and cable the drain all the way to main drain then repipe with a wye and cleanout using mission bands

I would say 99% chance it's a vent that someone...somewhere tied a drain into it. I would cut that leaking mess out (has to be done anyway) and go around and run fixtures in the house (one at a time) to see what happens. Those are not drainage fittings.

Thanks,
The disposal and sink both drain into a different vert c.i. stack 20ft away from here.
This could conceivably be the vent for disposal but it would mean it went vert from the disp ptrap stub then horiz then vert down to get below the floor joists and then 20ft horiz to this san tee.
in any case I don't see how it would be possible for the disposal water to get into this pipe
I appreciate your thinking about this problem
I'll try to get more pictures to show more of the system