I think I can see two seams, indicating a box-shaped construction or else that the top side is narrower than the under. A cushion from a boat cabin bench? We don't know about the opening? Has it been ripped open, or does/did it have a side zip, or a draw-string arrangement at the top?
I don't take the sheet to mean the man was sleeping in the bag. Perhaps he died on the sheet and it had to be disposed of like the body, or perhaps it was used to wrap the man to make it easier to get him into the bag.
I don't take the sheet to mean the man was sleeping in the bag. Perhaps he died on the sheet and it had to be disposed of like the body, or perhaps it was used to wrap the man to make it easier to get him into the bag.