The quiet kid in the back has noticed everything you thought nobody saw.

Being unobserved is mostly an illusion created by nobody commenting. Somebody who says little spends that time watching, and their picture of a room is usually more accurate than the picture held by the people performing in it. The watcher is not plotting anything. They simply had the better seat.

If you are the quiet one, your read on things is worth saying out loud. If you are not, go and ask them what they noticed, and take the answer seriously.