Red herring
Meaning: A deliberate misleading and diverting of attention from the real issue.
Origin:
Red herrings have come to be synonymous with the deliberate false trails that are the stock in trade of ‘who done it' thrillers. One theory has it that the meaning derives from the practice of using the oily and smelly herrings to lay false trails for hunting dogs. This practice is well documented from as far back as the late 1600s and Nicholas Cox's The Sportsman's Dictionary: OrThe Gentleman's Companion, 1686 describes it:
"The trailing or dragging of a dead Cat, or Fox, (and if necessary a Red-Herring) three or four miles... and then laying the Dogs on the scent."
The use of red herrings was a training exercise, intended to put the hounds on the scent rather than to throw them off it. Nevertheless, the laying of a scent trail for dogs does establish the linguistic 'surrogate' meaning for 'red herring' and the further step to 'deliberate deceit' isn't a large one.
Another theory is that the meaning derives from a trick played on one of his servants by the wealthy English clergyman Jasper Mayne. Mayne died in 1672 and willed large sums for the rebuilding of St Paul's Cathedral and to the poor people of his parishes. He also willed to a servant "Somewhat that would make him Drink after his Death", which was left in a large trunk. When the trunk was opened the servant was disappointed to find that the bequest turned out to be a salted herring. The will doesn't mention a 'red herring', but a report of the event in Jacob's Poetical Register, 1719, does, so we can date the 'false representation' meaning to that date.
Of the two theories, the Mayne story seems more compelling. It introduces the idea of a deliberate misdirection, which the other lacks.
Whatever the source, the figurative usage of the phrase was well established in UK by the early 1800s and had migrated to the USA by the middle of the century, as in this example from The New York Times, in May 1864:
But when the Emperor found that England would not join him in a war, he cleverly started the "red herring" of the Congress which he knew well enough was out of the question, but which has admirably answered his purpose of creating a diversion.
In Context: The detective discovered too late that the trail he was following was a red-herring!

Think About It: What other distinctive smells have been used in language to make a point?
Example: I smell a rat!