Why do the police stop chasing Montag?

As the lead player in this massive televised manhunt, Montag has managed to outwit his pursuers and make his way across the river. He's been helped in his escape by the fact that everybody was watching the police chase live on TV and so by looking through their windows Montag was able to see how close he was to getting caught. Wading into the river, he manages to throw the hound off his scent; he is then safe. As he drifts along with the current, Montag sees the police helicopters flying off into the distance.
Emerging into the chill silence of the countryside, Montag notices five men sitting round a TV screen. They are intellectuals, outcasts from society, now reduced to the status of tramps. One of the men, Granger, tells Montag that the authorities will continue their search, but in an opposite direction. The state simply cannot allow its authority to be challenged and so cannot openly admit that Montag has managed to get away. So the Mechanical Hound is unleashed upon a hapless passerby who just happens to be taking a late-night walk, a highly subversive activity in this dystopian world. The authorities can now proudly proclaim to the viewing public that the fugitive Montag, this dangerous threat to society, has been neutralized.