There are two secret police cars in the game. The first one is a Lotus Espirit which can be found in Free Roam mode. Go to the front of Snow Hill Police Station (yes, the one from the first game), then go down the hill. Turn right at the junction and then take the second left. It will be parked on the left. NOTE: This is the car from the fourth Chase mission.

The second one is a Renault Laguna estate. This can be found in Race mode (ROZZAS), but you cannot race with it. To use it in Free Roam, go to Mission 7 (The Jamacian). When you turn into Park Lane, go onto the wrong side of the road. Then take the first right. Continue up this road and take the next left to where the gunfight is taking place. You will see a burning police car on the right. However, DO NOT STOP. Instead crash into this police car and nudge it onto the main road. You need to do this BEFORE the helicopter arrives. Then when the car stops burning, 'simply' nudge it or drive it very slowly towards the nearest garage(Belgrave Square or Edgware Road).