Click the full stop
Photograph a page, shrink the negative until it is a millimetre across, and glue it over a full stop. The letter goes through the post looking like a letter. The reader knows which full stop, and has a microscope.
The FBI found the first one in 1941, on a letter carried by a courier. Somebody noticed a full stop that shone.
It is the purest example of this whole section: nothing is scrambled, nothing is coded, and the message is completely readable. It is simply too small to notice.
The thing on the wall
In 1945 Soviet schoolchildren gave the American ambassador in Moscow a carved wooden Great Seal of the United States. It hung in his study, and in his successors' studies, for seven years.
It contained no batteries, no wires and no power source of any kind, which is why every sweep for bugs missed it. Inside was a membrane, an antenna and a tuned cavity, built by Leon Theremin, the man who invented the instrument you play without touching it. Dead until the Soviets aimed a radio beam at the building from across the street. Then the membrane moved with the voices in the room, the beam came back altered, and they read the conversation off the difference.
Nobody hid a message and nobody enciphered anything. They hid the channel. And the counter-surveillance teams missed it for seven years because they were looking for the wrong kind of object, which is the same mistake as searching a page of cipher text for a secret when the secret is the dull letter beside it.
It was found by accident. A British radio operator stumbled across the audio on an open channel and heard an American conversation coming out of nowhere.
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Longer, harder and written for grown-ups. Worth a look when this page runs out.