Re: Adverse Possession of Shared Septic
Even if they don't have a recorded, written easement, the use, which apparently began with the permission of, or at least the knowledge of (with continued use indicating at least tacit, if not actual permission), it is quite likely that they now have a prescriptive use/easement which could continue until they do something that places an additional burden on the system above that they have previously placed on it, or until the septic system fails and needs to be replaced.
So while Newton is correct that AP is not an issue, it is quite unlikely that you would be able to prevent their future continued use of the septic system without their willing agreement to stop using it.
I'm a surveyor, not your surveyor & not an attorney.
Advice is general survey, not legal. Hire a local professional for specific advice.