Money is at stake.
Lying is part of the game.
Personally, I read up on the rules of etiquette at the particular casino I am enjoying (just ask, they all have them available in some kind of pamphlet), and I NEVER violate their rules.
As far as some kind of general, everybody knows rules of etiquette, my experience sounds about as gray as yours. I have played in dozens of live poker rooms in Las Vegas, Southern California
, and even a couple of cruise ships.
I would be careful with the information I have gleaned at any table. In your example, I would only disclose what I knew if I saw an advantage to me in doing so.