Unintentional Emanations and TEMPEST
463/480 Lecture, Dr.
Electromagnetic radiation can transmit secrets for miles: see TEMPEST
standards for radio transmission shielding. CRT displays are
notorious for leaking information, and even an unused VGA cable can
act as a passive transmitter capable of short-range FM broadcast.
Clever folks with telescopes have reconstructed ethernet packets
from the blinking pattern of the activity LED.
In 1945, the Soviets (at the time our allies) presented a carved
wooden Great Seal of the United States to the US ambassador.
In 1952, a passive
audio-to-radio converter was discovered under the eagle's
beak, later found to be a sort of passive bug like RFID.