Tues Nazir 66 (really on 65, which was yesterday)

If one found three dead in a field, we consider it a cemetery and you can't exhume them. If you knew that two existed and on e was found (or the other way around) as long as all three is not in the same category.

Tosfos asks why this should be so? Rashbam says, since this is all part of the Halacha L'Moshe Misinai and therefore it's not dictated by logic. Rather, it's just the Torah's decree that we don't deal with it as if it's an established cemetery.

However, Ri says it's very logical. if all know about the two buried there, if the third one was part of the original cemetery, we should have known about it too. Therefore, we need to assume that the one found was just a coincidence that it was buried next to the ones you know about. So either it's a cemetery you know completely about, or a completely ainchant rediscovered cemetery. However, it cannot be a hybrid between the two.