A guest at a hotel or bed and breakfast accidently sees into an open door in another room. He sees a human turn into a monster and devour another guest. The monster turns back into a human and is unaware the guest saw it all.
I believe that this could be T.H. White's The Troll that can be found in The Maharajah and other Stories (1981). The narrator is repeating a story his father had told him, about his fishing trip to Lapland, in the far north of Sweden.
Staying at a railroad hotel, he awoke from a nightmare wherein blood was seeping under the connecting door to the next room, and he felt compelled to look through the keyhole to ease his mind. Unfortunately what he saw was a troll eating a fellow guest.
His first thought was that he was going mad, especially when he sees that the resident in the next room is a mild-mannered professor; but the 'professor' gives him a brief glimpse of his real self, and informs the father that he will be next on the menu!
Father is so indoctrinated by the mundane world that he is literally incapable of dealing with this, and goes to bed as usual. Fortunately when the troll enters the room, he touches the fathers rosary, and is destroyed.