The Nazis had zero interest in keeping records or marking where the graves of their victims were, even during the time when they were winning or at least thought they were. When it became obvious they were losing, they made strenuous efforts to cover up mass graves, camps etc. They spent a lot of effort better used elsewhere trying to cover evidence, marching prisoners away from the front etc. From a strictly military standpoint leaving prisoners in place would have forced the Russians to devote some effort towards feeding them and providing medical care, if only for self protection against epidemics, roving bands etc. In some instances Geermans had bodies dub up and burned then the bones crushed to hide evidence.