In "Children Shouldn't Play With Dead Things", Dean reveals that Mary's uncle put up a grave for Mary shortly after her death. There is no body buried underneath. Sam initially wants to pay his respects, but Dean is reluctant to allow him. In "Alpha and Omega", Dean and Sam visit Mary's grave, presumably the same one, to pay their respects in case the world ends, and Dean's mission fails. Dean had hoped to be interred next to her. The purpose of this grave ends when Mary herself is resurrected by Amara.