In the Sherlock Holmes story, Silver Blaze, the key to solving a mystery turns out to be identifying what did not happen (and, as so often with things that do not happen, had therefore been ignored).
Gregory (Scotland Yard detective): "Is there any other point to which you would wish to draw my attention?"
Holmes: "To the curious incident of the dog in the night-time."
Gregory: "The dog did nothing in the night-time."
Holmes: "That was the curious incident."