at the scene vs on the scene
In your example of "at the scene," the subject is a passive observer, merely a witness to some notable action. In both examples of "on the scene", the subjects are active participants of the main events. The detectives and officers are working on the crime, while the new kid on the scene has taken 'center stage.' We don't know the fuller context, but the insouciance of this newcomer ensures that he has our attention. This kid has a lead role to play, he is not an extra just adding texture to the background.