There was an internal discussion about an unrelated topic recently, and it reminded me of an early experience in my career at Microsoft.
When I started, my 2nd computer was a pre-production PC/AT (the first was an XT). The AT had been announced by IBM about a week before I started, so our pre-production units were allowed to be given to other MS employees (since I had to write the disk drivers for that machine, it made sense for me to own one of them).
Before I got the machine, however, it was kept in a room that we semi-affectionately called "the fishtank" (it was the room where we kept the Salmons (the code name for the PC/AT)).
IBM insisted that we keep all the pre-production computers we received from them in this room - why?
Two reasons. The first was that there was a separate lock on the door that would limit access to the room.
The other reason was that IBM had insisted that we build a faraday cage around the room. They were concerned that some n'er-do-well would use the RF emissions from the computer (and monitor) to read the contents of the screen and RAM. I was told that they had technology that would allow them to read the contents of an individual screen from across the street, and they were worried about others being able to do the same thing.
Someone at work passed this link along to a research paper by Wim van Eyk that discusses the technical details behind the technology.