Continuity mistake: Danny drives up to the police station. Erin is waiting for him, leaning on the trunk of a police car. A different camera angle shows her standing up. Next shot she is leaning against the car again, then next shot she is standing.
Continuity mistake: Frank is talking to Garret in his office. A close up shot shows him sitting at his desk with his elbows on it. A wider angle shot shows him sitting up straight. Next shot shows him with his elbows on the desk again.
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.