Eavesdropping on a printer's signature

While doing a summer (1980?) co-op with Raytheon Submarine Signal division in Newport RI, a milk-truck-sized van pulled up out front and opened a sliding side door. Inside was a line printer that was busy printing out the same text that an internal (in the middle of the building, perhaps 150 feet away) line-printer was printing. There were mistakes, but it was readable. My guess is that with all the electromechanical pulses the printer was emanating this was pretty easy. Probably harder to do with a laser printer...