What clue indicates that Vera was not the last to die?

Vera seems to be the last guest of the ten on the island alive at the end of the novel. She goes back to her room after shooting and killing Lombard, the man she assumes was the murderer, as it was only the two of them left alive. In her room, she finds a noose dangling from the ceiling and a chair underneath it. Later, the police from Scotland Yard find her dangling from the noose dead, and assume she committed suicide.
The clue that shows she can't be the last one to die on the island is that someone has moved the chair she stood on to kill herself back against a wall: Vera could not possibly have done that herself.
We soon discover that Wargrave moved the chair back into place.