A “red herring” is a false clue leading detectives off the track during a criminal investigation.
The term comes from a practice once used to train police dogs.
When herring is smoked it becomes red, and when the young dogs were being trained to follow a scent, the trainers tossed smoked fish around to test their ability to follow a trail.
Escaping prisoners learned of the practice and often took red herring along to distract the dogs sent after them.
Misleading evidence is often called a “red herring”.