This is like playing roulette with the police cars. The police cars should be designed so that one can not tell the empty cars from the ones full of police officers. The aim is to put a lot of cars on patrol so that criminals feel threatened by both the empty cars and the cars that are occupied. Since most criminals are not likely to wait and find out if the police car is filled with personnel or just an empty car, such a method will make the patroled area safer. The policemen will seem to cover a lot more ground if the method is applied.
I would like to call this a scarecrow security approach. Just like animals do not want to land on a farm that has a scarecrow, because they can not tell the harmless scarecrows apart from humans, the criminals are repelled because they cannot tell the empty cars from dangerous ones.
This can also be used in security systems where there are not enough resources to cover the whole place. The technique can also be used at war, in cases where the soldiers are outnumbered or they get to be concentrated at strategic points but the details of their distribution are hidden from the enemy.