I believe the answer to today's problem is b6.
Firstly, black's only missing piece is his queen's rook, and this must have been captured on h4. This means black's b- and c-pawns have swapped over. The white knight must have sacrificed himself on c6 to allow the queens rook to escape. (Note, he cannot do this on b6, as the queen's bishop would block the escape route.)
As white is only missing one other piece, the rook we are trying to find the burial site of, it must have been used to get the c7-pawn to capture to b6.
None of the Royalty has yet moved. On which square was the missing White Castle buried?