Mystery pipes in basement - where do they go?
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)...