Milestones - S9-E12
Continuity mistake: Jamie is arresting a subject after he drops a knife. Jamie kicks the knife away from the subject and it lands next to the brown box and under the red wording on the box. In the next frame, the knife is now just behind Jamie's right knee. (00:05:10)
Answer: It's a common cop show plot device. If the police on the show always acted in the most logical way (i.e. not alerting a suspect to their presence) the show wouldn't have the opportunity for an exciting foot chase.