A sequel to The Collection by Lady Biggs.
Insipid little trolls, Snape thought. "Get out your textbooks and turn to page 1043," he said. No one moved. "Now."
Pages rustled immediately. He recited the contents of the chapter: "Sometimes it is necessary to disguise valuables as inocuous items in order to keep prying eyes from discovering them."
His eyes crawled over every tick, smirk, and gulp the class exhibited. Nobody revealed their guilt. Very well. "Today you will concoct such a potion. If successful, it shall reveal the item's true form as well as cloak it."
As the students began their mixes, Snape uncrumpled the piece of paper to detect any distinguishing marks.
In the corner was a Hogwarts crest with a "Teacher's Private Memo" circling it in fancy script.