Obviously from WWII and not relevant in the same way here and now - the inference was always "you never know who could be listening" - that word might get to "the enemy" about what you know....
It just occurred to me today, as much as preventing word getting to "the Hun" stopping people telling their neighbour about what "our Fred's best friend's sister's fiance" says is going to happen next was a morale thing. All the unsettling rumours can be so damaging to our state of mind.
That is far more relevant today. And I think people should bear the whole concept in mind when passing on hearsay and gossip....