In the 1970s, the Soviets managed to intercept top secret communications in the US embassy in Moscow and nobody could figure out how. While an antenna was eventually found hidden in the embassy's chimney, it took years to determine how what data was being collected for transmission and how. As a last resort, all equipment at the embassy was shipped back to the US for analysis. From IEEE Spectrum:
After tens of thousands of fruitless X-rays, a technician noticed a small coil of wire inside the on/off switch of an IBM Selectric typewriter. (NSA engineer Charles) Gandy believed that this coil was acting as a step-down transformer to supply lower-voltage power to something within the typewriter. Eventually he uncovered a series of modifications that had been concealed so expertly that they had previously defied detection.
A solid aluminum bar, part of the structural support of the typewriter, had been replaced with one that looked identical but was hollow. Inside the cavity was a circuit board and six magnetometers. The magnetometers sensed movements of tiny magnets that had been embedded in the transposers that moved the typing “golf ball” into position for striking a given letter.
Other components of the typewriters, such as springs and screws, had been repurposed to deliver power to the hidden circuits and to act as antennas. Keystroke information was stored and sent in encrypted burst transmissions that hopped across multiple frequencies.
For more on this fascinating story, check out former intelligence officer and technologist Eric Haseltine's new book: "The Spy in Moscow Station"