Not surprisingly, Oreki had it figured out like 2 episodes ago and blackmailed the criminal to help sell anthologies. Satoshi seemed upset, but went along with the final act to help Tanabe Jirou, AKA “Juumonji”, pull off the “ko” theft even with so many spectators waiting for him show himself. A little chemistry latter and Jirou burns the theft in front of everyone, completing his prank/message and clearing out all but four remaining anthologies.
This mystery makes it obvious that Hyouka isn’t built to allow the viewer to solve mysteries before the characters, it’s meant to be about the characters. That being said, Hyouka didn’t leave out clues, it just didn’t cover trivial information that would’ve either tipped the viewer by making suspects too obvious, or bored them by burying the trivial information beneath more trivial information. Though I do think Jirou’s plan to send a message to an old manga partner was a bit far-fetched and had little to no chance of succeeding. Barring straight up telling the person about this prank/mystery, I don’t see how a transferred student would pick up on it. Also, the related drama within the manga society really seemed forced toward the end, where the other members just decided that they’re going to eternally disagree and not get along. Makes all the events beforehand seem pointless if they’re just going to end it by acknowledging that they understand their differences, but choose to ignore that they do.
So Oreki has to keep a secret from Eru now, since she’d probably get upset that such a mysterious mystery ended in such backhanded tactics. Oreki is going to have to either get Eru to forget about the Kanya Fest mystery (unheard of, thus far) or distract her with a new mystery to legitimately solve.
I’m a little disappointed that it wasn’t actually Oreki’s sister, but instead some irrelevant backstory.