 You know, this is from a Sherlock Holmes, I forget which one of the Sherlock Holmes stories, but essentially Holmes recognizes who must be the culprit because a dog didn't bark. The dog barked at everyone he didn't know, but the dog was friendly with one particular person and didn't bark. So that person must have been responsible because the dog didn't bark. It's a telltale thing when you expect something and you don't hear it.