why did the hound threatingly extend its silver needle toward Guy Montag?

At the beginning of "Fahrenheit 451," the first time that we are introduced to the hound, it expresses a rather disturbing and illogical hostility towards Montag. It is just a machine, one who has to be programmed with DNA or some form of scent or fluid of a person before it learns to "hate" or hunt or stalk or destroy anyone. And yet, as Montag comes into the fire station at the beginning of the book, "the hound growled" and Montag

"saw the silver needle extend upon the air an inch...the growl simmered in the beast and it looked at him."

The hound, a machine, seems to instinctively consider Montag to be an enemy, and is even stepping forward menacingly as Montag slips upstairs.

This event actually foreshadows the hound's chase of Montag at the end of the book. Indeed, Montag does turn out to be the enemy, and it chases Montag through the city streets, intent on the kill. Perhaps it did sense Montag's defection even before he himself realized it, who knows.