There's nothing in the room, but a DC 35 Perception check, or a DC 25 Survival, will notice faint hints of footprints in the well swept floor; even the best cleaning can't keep out all of the ever-present sand. And, more tellingly, they'll reveal where they stop. A careful inspection at that point reveals what looks like a perfectly smooth floor, but a DC 19 Will save while touching it allows her to disbelieve the illusion over the trapdoor in the floor, at that point.