Plumbing – 2 parallel drain pipes

Here's my speculation, based on your description and my experience. It is just a theory. Do not commit any resources based on this without collecting corroborating physical evidence supporting it. A direction to investigate at least.

The main floor plumbing goes out to a septic tank. The pipes were not placed lower because there was no need, it just meant digging deeper outside. No one wants to do that without good reason.

When the city sewer was installed, the basement construction tied into it, but the old plumbing was never redirected, it remained on the septic tank. Not the best scheme and possibly illegal, but I'm not surprised someone did this.

It is highly probable there is nothing below to the basement to tie into. The sewer may not be much lower than the basement drainage exit. Anything placed lower than this will likely need to drain into an ejector pumping chamber.

Thanks everyone for the feedback and tips! Turns out I was able to get a clear answer by calling the sewer company:

1) They looked at their recent scopes and confirmed that our property does indeed have two sewer laterals. They also observed some corrosion and decided to come out and take a look.

2) They brought a truck out to our house and performed a sewer scope - entering from the manhole immediately downstream and running upstream to where our laterals connects to the main line.

3) They then had me run water from every fixture in the house, one after the other. Turns out that all fixtures run to one lateral, confirming that our basement pipes do indeed tie into it somewhere out in the yard. The sewer guys theorized that what the inspector mistook for an exterior cleanout was actually the basement pipes tying in! Our second lateral is "inactive," but is likely an old stormwater drain for our back patio...which may or may not be legal.

Thanks again for the suggestions. Glad we got this mystery figured out! Now to determine how old this place is (we have aerial photos proving the home was there prior to the build date on tax records)...