Your question and dilemma is one that is a good lesson for people. The sanitary code is in place for builders and homeowners to follow to avoid contamination and potential health problems, thus the purpose of tank sizes, distance between water and sewer, and proper installation and maintenance.
Two important factors would be:
Home buyers should always obtain a home inspection, including a septic inspection when it is a private well and septic.
Home Owners should always pull permits when doing any home improvement to their home.
I am sorry I do not have the answer, let alone a solution that is not potentially a costly remedy. However, from a Realtor's perspective and how this affects the outcome of the sale of your home, this is a material issue that needs to be disclosed. Depending on the source of this information, another opinion could be sought to assure accuracy of the condition. The Oakland County health department may have records on file if the home was built in the last 20 years or so. If it is an older home, built prior to 1980, they may not be able to provide information, therefore, you are left to rely on well and septic inspectors to determine the condition. Although it is not a requirement in Oakland County, there are some counties that require inspections of these systems and certification of that passed inspection prior to selling the home. However, buyers should always have an inspection regardless of the requirements.