The History of TEMPEST
One never knows how and where information may be encoded.
Take the case of an Bell telephone engineer back in the early 40s. He noticed that an oscilliscope seemed to spike every time the brand new, fancy, and highly top secret encrypted teletype machine coded a letter. He figured out that, if one studies the spikes, they could read the plain text the machine was encrypting.
And thus was born TEMPEST, the US Government's top secret method of gathering information based solely on the electromagnetic waves that all electronic devices give off. Newly de-classified documents recount the history of this still highly confidential information gathering system.