Usually nothing is said, but it is for them.
Probably where you saw the "secret doors " thread was on another site. They are always talking about it, but not one has the foresight to tell what it really is.
I am sort of the straight laced, follow the rules type of person, so I generally try to advise the same.
I do see some serious situations with these secret places. If it is so secret, and there is not much traffic, and the crew knows about them, then what stops them from doing something to you or family, and it is your word against theirs.
Also, most of these places prevents passengers from getting to the front of the ship, so they can stand on the rail, and proclaim they are too "King of the world" (Titanic movie)
These threads are a good example of when board moderators do a huge disservice to it's members by not pointing this out. (Not Cruisemates)
I know, I know, I sound like a mom, but I would hate for something to happen to any one of you. Cruising is such a fun thing to do. It should also be safe.