As some of you might know, sometimes if you go down into the cellar in chapter 2 or chapter 3, you will hear a woman crying. You then go into the room where the chain is and you (the player) a ghost hand, although Adrienne does not seem to see it. Adrienne then rations this to be a rat. It was clearly not a rat. Rats don't cry like that, and they certainly don't have ghost hands. It's gotta be one of Carno's wives. But is there a way to find out which one? Every time I play the game, Adrienne just rations it to be a rat and that's the end of that.
If ti was one of his wives then the only one I can imagine it being is Leonora. Hortencia, Victoria and Marie are impossible, in all three cases the women were free and able to go about their business when the murder happened, none would still be there had they been locked up previously. It could be Regina, but I doubt is ability to get a woman of her obvious size from the cellar to the 1st floor to kill her, Leonora looks a much more slight woman to drag around.
There is another theory though that the cellar woman is someone else and pre-dates Carno. The theory goes that the demon is actually attached to the house and not the book directly, the book just acts as a conduit to allow it into the world and that at some point in the past the demon was free and captured only to be released by the book once more. Or it could be the demon was the soul of the torture victim (or Carno's daughter, her death could have been neglect rather than straight murder)