When an older person or Talmid Chacham walks into a place where you are sitting, you must stand up for them (or least "rise" a little from one's seat.) - see Yoreh Deah 244:1. If someone was sitting in a room and noticed that there was an older person now sitting in the room as well that wasn't there before. Presumably they had just walked in and the person didn't notice.
Must the one sitting now stand up or rise a little in order to honor this older person that had previously walked into the room, but is currently sitting down?