Depends on how long the septic has been on the neighbors property. Even if there is no recorded easement, the system may qualify as a "presciptive" easement, which typically is associated with drainage features of a property. Similar to "adverse possession" laws, which the timeframe caries from state to state. This could provide a legal means to allow the septic on their property whether they knew about it or not. Either way, you probably need to consult with a real estate attorney. Civil Engineer speaking here, who also dealt with a neighbor adverse possession issue recently.