When visiting that family burial plot, pay close attention to a “lone burial” of a “non-relative” close to your “known” relatives. That lone burial may actually be buried in the family plot (sometimes those records aren’t available) and may actually be a relative.
Always worth it to get pictures of adjacent stones that you don’t think are related and to get an overview picture of the known family graves–including the nearby ones that “aren’t related.”
But that lone burial may be just that–a lone burial. In cemeteries where plots are expensive, your relatives may have sold that unused spot to someone outside the family.