It is evidently possible to determine what someone is typing on a keyboard purely by painstaking analysis of tiny differences in the sounds made by the keys. A research team used the standard letter distribution in English to reconstruct what had been typed by a typist using a computer keyboard, using just a 15-minute audio recording. [This is a creative application of a standard cryptanalysis technique.] Perhaps quiet keyboards and background noise should be considered information security measures?
More physical security resources