I first read about it in an old Tom Clancy novel. A short bit of googling led me to this: http://en.wikipedia.org/wiki/Canary_trap http://en.wikipedia.org/wiki

I first read about it in an old Tom Clancy novel. A short bit of googling led me to this: http://en.wikipedia.org/wiki/Canary_trap http://en.wikipedia.org/wiki/Canary_trap So the technique was well known by 1987, which certainly qualifies as typewriter days. Apparently it was used in fiction as early as 1972.