Minho, Newt, Thomas, and Teresa retrieve the Maps that were hidden in a secret storage closet at the back of the weapons room.
Minho can't believe that Thomas and Teresa might've thought of something that had never occurred to any of the other boys in the two whole years they'd been in the Glade.
They present their theory: maybe they weren't supposed to compare each section one day to the next, but look at all of the sections one day at a time.
Thomas realizes that even if they had tried that method, maybe they just weren't able to see it.
A giant light bulb appears over Thomas's head, and he requests that they gather all of the wax paper, scissors, black markers, and pencils that they can find. His goal is to trace the Maps onto wax paper so that if they pile up all of the Section Maps from one day and look through the stack they might see something.
The first time he tries it, he strikes gold: slightly darker than the grid lines is the letter F.