An element commonly used in traditional mysteries is a device called a red herring. Sometimes the term "red herring" refers to a plot point; usually it refers to a  person.

A red herring is a false trail. Until recently, the accepted etymology of the idiom was that red herrings were used to train hounds to track scents. This seems to be false etymology, most likely intentionally introduced as a prank that defines the idiom by example: a false trail within the etymology of an idiom that stands for a false trail.

Apparently, the term was first used in a story by William Cobbett (1805), in which he claimed that as a boy he used a red herring (a cured and salted herring) to mislead hounds following a trail; ultimately, the story evolved to refer to a method of training hounds to follow an underlying scent—not be distracted by a secondary scent. The story served as an extended metaphor for the London press, which had earned Cobbett's ire by publishing what he deemed false news accounts regarding Napoleon.