Ibn Ezra points out that the Hebrew uses the definite article – not “a” but “the” coffin – i.e. the coffin he had prepared for himself.
Many years later, say the sages, when the departure from Egypt was imminent, Moses spent three days and nights vainly hunting for this coffin.
Eventually, Serach bat Asher, the last survivor of Joseph’s generation and by then a very old lady, met Moses and asked why he looked so weary.
When Moses told her, she took him to the Nile and said that when Joseph died the Egyptians, knowing the Israelites could not leave without Joseph’s remains, put Joseph’s coffin in another, leaden coffin and sank it in the Nile.
Moses thereupon took Joseph’s cup and cut four plates out of it. On one he engraved a lion, on the second an eagle, on the third a bull, and on the fourth a human figure. He threw the first into the water and implored Joseph to show himself, but without result. He repeated this procedure with the second and third plate, but still nothing happened.
Finally he threw in the fourth plate and implored Joseph to come up, and this time the coffin rose to the surface and the Exodus could proceed (Midrash Ex. Rabba 20:17).
It rarely happened in Jewish history that a family could take their ancestors’ or dear ones’ remains with them when they moved from country to country.
Tragically, the sacred spots where Jewish families had buried their dead were often desecrated by a heartless enemy who had respect for no-one, neither the dead nor the living.
This is all the more reason to ensure that wherever we are we conserve and preserve Jewish cemeteries and never let them decay or disintegrate.