As far as I'm aware this phrase originated as one of many used on
propaganda
posters during
World War II. It was basically a
reminder to
civilians involved in the
war effort to be wary about what they
spoke about, or to whom, for
fear of information getting into the
hands of
enemy spies. I remember seeing a poster (in a
museum, I'm
not
that old) with this
slogan imprinted across it, and and
a
picture of two young women chatting about something they'd
overheard, whilst behind a nearby
wall a stereotypical-looking
German was hastily scribbling in a
notebook. In other words, even if
you think you're
alone and can't be overheard, you probably can.
Modern surveillance equipment makes this saying even more apt,
and it was taken to its literal extreme during the Cold War. The
Russians very generously offered to build the United States a lovely
shiny new embassy building in Moscow in the 1980s. Obviously the
Americans were suspicious, but their engineers oversaw the entire
construction and after completion the entire place was carefully
swept to ensure there were no bugs or other listening devices.
Everything was clean, until by chance someone rested a bug detector
against a wall. The KGB had embedded highly sensitive
microphones-cum-transmitters into the very concrete used to build
the walls, which were found to literally have ears...