One point when Montag questions the limitations on his freedom comes early, when he visits the firehouse and the Mechanical Hound growls at him. The Hound is set up to recognize people by their DNA, described as "amino acids and enzymes," and normally it shouldn't have any reaction towards any person it isn't specifically programmed to kill. Montag realizes that he is not safe, that anyone with the knowledge could program the Hound to go after him.
"This isn't the first time it's threatened me," said Montag. "Last month it happened twice."
"We'll fix it up. Don't worry."
But Montag did not move and only stood thinking of the ventilator grille in the hall at home and what lay hidden behind the grille. If someone here in the firehouse knew about the ventilator then mightn't they "tell" the Hound...?
(Bradbury, Fahrenheit 451, Google Books)
Montag has always considered himself above the law, outside of the purview of his own work, since he represent the forces of "order" keeping the populace safe from the "chaos" of books. Now, remembering that he is secretly keeping books in his house, he becomes scared; Montag realizes that his little secret isn't so safe after all. Someone might know, someone might go after him. If Montag isn't even safe from his own organization, than is he truly free? Or is society as a whole just a place where people exist and then die for no higher purpose?