Hyouka: Motsubeki Mono wa

Was bundled with the third volume of the manga.

It's another regular day, with Houtarou Oreki sitting at home as usual; that is until his sister Tomoe ropes him into working as a lifeguard at the local swimming pool. Upon reaching the pool, Oreki coincidentally meets the other members of the Classics Club. Eru Chitanda notices that a white object that was on a woman's ear a while ago suddenly disappeared, which leaves her curious about the mystery behind it.

Hyouka: Motsubeki Mono wa features Oreki and the rest of the Classics Club as they have fun at the pool and solve the mystery that has piqued Chitanda's curiosity.

(Source: MAL Rewrite)