Continuity mistake: Part way into the episode they show the front of Jamie's Ford police car and the car number on the front plate, but when they pull up to the court house to talk to his sister, it's a Chevrolet police car with a different number on the front. (00:09:05 - 00:09:40)
Character mistake: Commissioner Frank Reagan refers to Lt. Gormley as Sgt. Gormley in this episode. (00:08:30)
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.