“The dog that didn’t bark” (or “the dog that hasn’t barked”) is a reference to Sherlock Holmes, specifically the short story “Silver Blaze” by Arthur Conan Doyle. In Silver Blaze, Holmes solves the mystery partly by noticing something that did not happen: The watchdog did not bark during the theft of a racehorse. This silence tells Holmes the culprit was someone the dog recognized—one of the key clues that cracks the case.

(http://www.autoadmit.com/thread.php?thread_id=5796806&forum_id=2Elisa#49423636)