“They Heard it Through the Grapevine”
I’m sure there’s more than one definition of a tight-knit neighborhood, but here’s mine: when a friend calls you up to congratulate you on selling a home two blocks away from him — and he doesn’t know the Buyer (your client), and the home hasn’t even closed yet!
It turns out that he heard about the (pending) sale from a next-door neighbor of the Seller.
P.S.: Buyers and Sellers can tell whomever they want, post-closing, but it’s a good idea for Realtors to keep their mouths shut.