[icerabbit]

Maybe they had to make a repair?
Maybe they took out a floor drain?

Where does the upper end of the pipe go to?

 

[icerabbit]

If no water runs through some of the pipes, then they are vent branches / loops to aid with drainage.

Along the lines of:

 

[icerabbit]

As long as you have enough clearance to enter the waste line from above, I don't see why you could not take advantage of that hole, if it is in a suitable location layout wise.

If the clearance is tight, there exist shallow depth and / or length shower drains to help install a trap in confined spaces. Confined space shower drain or trap should yield some search results.

 

[icerabbit]

Same. It ought to be there for a reason. Some more digging and poking around is in order.

And, again the lines that go vertical then horizontal part way up the wall, group together are the vent stack lines, ensuring proper drainage and flow of the waste lines, by letting air in from the roof area. They possibly tie in somewhere with the bigger main stack.

Maybe through the realtor, or directly, reach out to the previous owner, to ask about the purpose of the hole, how, what, why, ... what the intention was?