Researchers have found yet another way to eavesdrop on a computer user. A dot-matrix printer, still used to print multi part forms, gives out distinct sounds as each letter is formed, and computer software has been developed to "read" the sounds with a high degree of accuracy. Background noises can be screened out. This is one of several techniques developed in the last decade that allow useful information to be extracted from seemingly meaningless sounds. Intelligence agencies are always working to increase the number of tools they have to make sense out of seeming nonsense.
For example, five years ago, a technique, based on the sound that is made when a user strikes a key on a computer keyboard, enabled you to determine what was being typed. Collect enough of these key noises, and based on what language the typist was using (all languages have a certain frequency of letter use), you can quickly decode those key noises and figure out what is being typed. This sort of predictive analysis is nothing new in Cyber War. This works for email or IMs (Instant Messaging). You can also positively identify different email users by analyzing their text. That same technique is used to crack secret codes. One of the oldest (by several decades) of these computer eavesdropping techniques is the ability, using fairly simple equipment, to pick up the small electronic signals your keyboard makes every time a key is hit, and analyze those to figure out what is being typed.
Most of these techniques, however, assume you can get pretty close to the keyboard in question. Electronic signals from keyboards are kept from going far by modifying keyboards. These are the U.S. Tempest grade keyboards often required when you are doing classified work. Getting a recording device near a keyboard may also prove difficult. So while the spies keep coming with great new tools, you still have to be at the right place at the right time to make it all work.
But not always. One of the oldest (by over 150 years) of these techniques was discovered by accident. In the early days of the telegraph, experienced operators found that they could tell who was on the other end of a telegraph line by the rhythm of how the telegraph key was hit. When computers came along, it was possible to automate that particular intelligence gathering task. Telegraph, via Morse Code, is still used in some intelligence work, because you can get the message through with modest equipment resources.