My first course of action would be to find the septic system if you think what you posted is not it.
There are many configurations of septic systems and especially engineered ones.
I needed a engineered system on land in the Colorado mountains that didn't look like any septic system I ever saw.
You might also want to check and test the well water system as you could get burned there.
Bear in mind you never know what a homeowner may do to anything when faced with a problem they really don't understand.