It could mean two other things. (I didn't see the series)
1. Seppia's body is not in the room, but traces (small things) of her body are left, such as blood. In that case you could say 'cleansing any traces of Seppia's body', or removing any small clues that Seppia's body was there.
2. 'Cleansing' can mean a ritual cleaning of a body, performed as a rite in many religions after death. This would be if Seppia's body was left in the room where whatever it was that was 'secreted among her things' was found.