Espionage by CRT mirroring
After searching a bit more thanks to Eugene's comment, I found this, where it states it can be done from hundred meters, even without very expensive equipment.

See Wim van Eck's legacy.

Fragment:


  Oscillating electric currents within your monitor produce radio
  frequency electromagnetic radiation (EMR) that correlate to what the
  monitor displays. In cooperation with the BBC in February 1985, van
  Eck was able to confirm through experimental proof of concept that
  this form of electronic eavesdropping is possible from distances of up
  to several hundred meters.
  
  While such danger to information security was already known at the
  time of van Eck's paper, it was generally believed that such
  eavesdropping was prohibitively difficult for amateurs — meaning, for
  the most part, non-military personnel — and would require extremely
  expensive, specialized, restricted equipment. Wim van Eck's research
  showed that it can be accomplished with nothing that isn't readily
  available on the open market — that, in fact, "In the case of
  eavesdropping on a video display unit, this can be a normal TV
  broadcast receiver."