How did they happen to find the cipher in "The Hollow Needle"?

Question

In The Hollow Needle (1909) by Maurice Leblanc, did M. Brédoux just "drop" the paper with the Hollow Needle cipher?

Maybe I missed something in the book, but the way the good guys and Beautrelet just get to stumble upon that very important piece of information feels weird to me.

It enters the narrative like this (see page 76):

At that moment, the sergeant of gendarmes came up to M. Filleul and handed him a crumpled, torn and discolored piece of paper, which he had picked up not far from the place where the scarf was found. M. Filleul looked at it and gave it to Beautrelet, saying:

"I don't suppose this will help us much in our investigations."

Isidore turned the paper over and over. It was covered with figures, dots and signs [...]

I mean, it's a legit, massive secret paper that have been around and hidden for centuries and Brédoux or Lupin (or some of the Lupin-guy) just dropped it?

Answer 1

I also asked that question on Reddit r/books and here is u/VladimirOo's answer :

Then, Lupin's motives are never that clear in the novel. He might have made a mistake by accidentally dropping the paper - after all, he rarely got challenged like in this story.

My guess is that he did it intentionally to test out Isidore's abilities. When the hollow needle's secret is uncovered, he had already given up on his thieving life and is ready to pass its property on. So dropping clues and mysteries behind him might be done on purpose.