
Continuity mistake: When Paul Walker is talking on the phone to get medical help, the phone changes hands instantly from the right to the left without time to do it. (01:24:30)

Continuity mistake: Close to the end when we see Brian in his orange car speeding down the road, if you notice Leon parked his car on the side of the road, but when it cuts to a front view of the road when we see Brian's car pass the camera, Leon's car has vanished. (01:21:25)

Continuity mistake: When Brian has picked up Dom from the first race, the Chinese guys drive up behind him. If you notice out of Dom's window it is a blue motorbike that appears, in the following shot the motorbikes on Dom's side are both red. (00:25:40)

Continuity mistake: The smudges and blood-stains on Dom's shirt in the end change a lot during the scene. (01:25:50)

Continuity mistake: When Brian pulls up to Dom's garage towing a red car, Jessie looks under the hood, and the shot from inside the car shows Jessie in one shot with his arms down, in the next shot, he has them up or crossed. (00:37:25)

Continuity mistake: When Brian drives up to the street racers meeting in the alley, there is a pipe of some sort hanging from his engine. When he picks Toretto up, it's no longer there. (00:10:00 - 00:24:00)

Revealing mistake: When Brian spins the Supra to go after Dom at the end, there are tyre marks from previous takes visible. (01:31:00)

Continuity mistake: At the end, as the crew go for the last hijacking, we see all the cars driving over a bridge. However, the railings that are on the side of the bridge are not visible when we see the inside of Leon's car. (01:17:40)

Continuity mistake: In the first hijacking, the window is pulled out and the rubber seal hangs from the lorry. Camera cuts to the next harpoon and the seal has gone. (00:01:30)
Answer: There are two ideas of why the police did not come around. Firstly they might have found out after the car chase that Brian was an undercover cop so it would be more effective to allow him to try and discover information instead of going around and arresting Dom. The other idea is that the police might not have enough to arrest him. Dom was not the one driving when they found him and other than racing, it might be tough to arrest him.
Lummie ★
Cops are usually completely unaware of an undercover cop's identity. The more, realistic reason is that the cop didn't ID Dom and since Brian's car was later destroyed, there's nothing for them to investigate. Afterwards, they might ID Dom, but there's an investigation on him, so they'd leave him alone.