The excitement is unavoidable. That's what Erik wants. A witness to his plans. Defeating Reinhard directly at the same time showed the audience what he really was.
However, while in the hallway where the commotion was taking place, Ashnard didn't see Erik anywhere. There's only Arlon. It made Ashnard think that Erik was hiding while Arlon was his public puppet.
Ashnard thought that Reinhard's plan was riskier as it would face Arlon and Erik head-on. He didn't agree, but he couldn't change Asberion's mindset either.
That's what makes Ashnard have to think of another way. He then decided to invite Gerlon to look for Erik. Better to prevent it before the climax of Erik's plan occurs. And his destination was none other than Erik's room.
With his prefect access, he knows the location and has a spare key for Erik's room. He turned one of the keys and unlocked the door. Since it was night, and all the students must have come to the arena, there was no one inside.