Plumbing Mystery - Question

[Derek L.]

I have a house under contract and during the inspection, it was noted that the exterior clean-out was missing a cap. Not a huge issue or fix. However, after visiting the property, I noticed that on the ground by the clean-out was a 90 degree elbow joint and longer pipe that ran down the side of the house and terminated about 15' away.

The clean-out is situated on the same wall as the kitchen sink and water would come out when the kitchen sink was running. My initial thought was that the plumbing had been routed outside of the house for the reasons below, but I think that's unlikely. Here are some observations / questions that I'd like opinions / theories on:

• If this is just a clean-out, why was there a pipe connected to it at all?
• I'm curious why someone put landscaping stones around the clean-out; looks like whoever had the property was accommodating this setup despite it being against code.
• The clean-out was threaded and accepted the cap (which I found nearby) which tells me it was probably originally intended to be a clean-out and not a re-routing of the kitchen plumbing.
• I ran the sink with the clean-out re-capped and nothing backed up, further confirming the point above (hopefully).
• As I mentioned, the inspection turned up no further plumbing issues, but I'm still curious about the first question.
• I've included a few pictures (it's been re-capped in some) below so you can see what it looks like.

Take a look and let me know if this is something you guys have ever seen before.

Thanks in advance!

Derek