Bowser had been spending much of his time trying to learn about the various other dimensions, in hopes of finding a common thread, a hint at an answer as to why they were here, and an answer on how to leave. So during the forest's "night" phase when he sees the notice, he collects the case he Regained from his apartment, then heads to room 301.

Once the Koopa King is there, he knocks on the door. If and when the Exarch arrives, he's leaning against the wall opposite to the door, holding the case. "I saw the message in the lobby. I'm here to trade information."