Spies and secret agents often have to be cryptic in their communication, using code words or circumlocutions to refer to things best not said in public or within earshot of potential enemies. Code words work well, because they can be completely unrelated to the subject matter at hand. However they have the disadvantage that they have to be agreed on beforehand, and both parties need to remember (or be able to look up) what the code words mean. Another sort of secret communication is to ad-lib a message on the spot, using subtle metaphor that most people would overlook.

The best ad-libbed secret messages are ones where the intended recipient can work it out from context, but nobody else. For example, this line spoken by James Bond in You Only Live Twice:

Little Nellie got a hot reception. Four bigshots made improper advances towards her, but she defended her honour with great success.

Or there's the bad sort, like this from Star Trek II: The Wrath of Khan:

SPOCK: Admiral, if we go by the book, like Lieutenant Saavik, hours could seem like days.
KIRK: I read you, Captain. Let's have it.
SPOCK: The situation is grave, Admiral. We won't have main power for six days. Auxiliary power has temporarily failed. Restoration may be possible in two days ... by the book, Admiral.

Which is completely transparent to everyone, except apparently the main villain of the movie, Khan, who, despite being a genetically engineered super-genius, is completely bamboozled when the Enterprise regains auxiliary power two hours later.

Transcript

Fozzie: Did you just say "bigger fish to fly"? Are you one of us?
C-3PO: What? I'm a droid!
Fozzie: Indeed, I can see that. But are you in the know?
C-3PO: I know many things, including over six million forms of communication.
Fozzie: Good, good. We just need to collect a few last minute things, and we'll be ready. And remember, my code name is "Fozzie".
C-3PO: I see. Okay, then.
R2-D2: This smells a little fishy. And not just because of the fish.
C-3PO: You know, I find that "Fozzie" creature to be a little odd.
R2-D2: Ben being odd instead of strait-laced is definitely a sign that something weird is going on.