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?