POLL: What is this pipe under my slab?

[Soparklion]

House ~1945, full basement and 2 stories. I am replacing a section of cast iron pipe (long story). The waste pipe descends from the 2nd floor to 1st floor (no connections) to the basement in the far left corner in the photo, which is no the corner of the house. In the photo, there is a wye about 30" from the wall, which runs parallel to the outer wall. There is no basement drain on that side. There is no other plumbing on that side of the house. There is an iron pipe at level grade at the corner of the house ~10' from the picture that used to accept the downspout. Plumbing a downspout to the sanitary sewer was a common practice in this area decades ago. Mine has been appropriately rerouted to drain 8' from the house. There is a full bath in the basement if that makes a difference, it is off to the right.

1. Is that a wye to a vent stack? I thought that my vent stack started higher.
2. If the wye doesn't lead to a vent stack, where does it go?
3. If the wye isn't a vent stack, what do I do with it? My camera shows that section to be in good shape and I'm tempted to Fernco to the wye on both sides without taking out the cast iron wye or touching the mystery pipe.

Pic below with black arrow indicating location of waste pipe from above in the corner; red arrow is direction of the mystery pipe. The black corrugated pipe is my interior French drain.

Picture

 

[Jeff H Young]

You're the one tracking it down if it doesn't connect to anything you don't need it .