Earlier and after Artemis had sealed the book away, an explosion occurred. It wasn't large or threatening, but it had caused a stray object to fly in through the glass of the compartment he and his fellow Slytherin were staying in, obviously breaking the thing and hitting the cage of Jersey. Easily fixed, but Casimir felt the need to go and have a chat with whoever caused the explosion, just to make sure it wasn't done purposely. He cast a spell to repair the window and told Art to wait for a while... or follow if he wished. He picked up the foreign object that busted their window before heading out.
After following the direction where the stray projectile seemed to have flown from, Casimir arrived at a room full of Gryffindors and a few Slytherins. The bubbly Samantha, the timid Willow, and the stone cold Acacia were there, just to name a few. He made his presence known by partially entering the compartment and leaning on one of the doorposts.
"Sorry to interrupt whatever joyful meeting you're all having here, but would someone care to explain why an explosion occurred, allowing THIS thing to break the glass of my compartment?"
Casimir held up the foreign object for them to see.