From: Rabbi Moshe Sofer
To: Rabbi Eliyahu
A Kohen entered a non-Jew's courtyard containing a gravestone inscribed over 415 years earlier, with a family tradition of a former Jewish cemetery. The Chasam Sofer permits Kohanim to enter, relying on three cumulative grounds for leniency: the bones may have decomposed, may have been removed when Jews left the area, and the ground retains its presumption of purity since the gravestone does not conclusively prove burial at that precise spot.
Source: Shu"t Chasam Sofer YD §337