At the audiations for Noirebug for Clara's video, Adrien is there to take picture (with his trusted camera) of it all as well audition for an extra. When Clara does a secret test of character which Marinette passes and is sent to be Noirebug. When Chloe arrives and auditions for Noirebug only to be told that the position is taken until she tells of her idea for there to be more than one person playing Noirebug which Clara takes as to make everyone (including the boys) in the video Noirebug to represent that anyone/everyone is a hero. That's when Adrien leaves, not wanting to spoil his secret identity.
