Unlike the others, who were somewhat startled by Elyse entering, Oner's ears weren't merely decoration. They actually gave him a sense of hearing stronger than a human's, so he had heard the girl sneaking into the room, not startled by her when she suddenly shouted. Plus it also helped that Oner had the door in his general field of vision as he was talking to Belle, so he was able to see the little girl sneaking in. After Belle spoke to her Oner also decided to ask her a question as well. "Elyse, I was wondering if you had seen my father, Serin. Did he come with you and the others?"