In episode 12 of season 6, Monk guesses that most of the product of the theft is still in the bank, in one of the boxes in the vault. So far, so good, I could follow his reasoning.
But I did not understand how he guesses the exact number of the box. What is his reasoning on this precise point ?
A posteriori, when checking the card of that box in the files, the phony name it is registered to, namely J. A. Guar, confirms his guess.
But he checked only that particular card, he did not have to go through all the files. How come?