Why did the mechanical hound puzzle Montag when he returned to the firehouse to turn over a stolen book to Captain Beatty in Fahrenheit 451?
Montag is puzzled because the Mechanical Hound is gone when he goes to turn in the book.
Montag finds the mechanical hound very unsettling. One of the things that he does not like about it is that he seems to be unable to classify it. Is it a machine or a living thing? Is it alive or dead? It seems to be a paradox, both at once.
The Mechanical Hound slept but did not sleep, lived but did not live in its gently humming, gently vibrating, softly illuminated kennel back in a dark corner of the firehouse. (Part I)
Montag describes it like a bee, and when he goes up to it the beast growls at him. He is both curiously drawn to it and frightened by it. It almost seems to attack him, extending its “silver needle” at him, where it keeps its poison. This scares him even more.
Later Beatty laughs this off, telling him that the beast could not be targeting him.
"Montag . . . ?"
"It doesn't like me," said Montag.
"What, the Hound?" The Captain studied his cards.
"Come off it. It doesn't like or dislike. It just `functions.' It's like a lesson in ballistics. It has a trajectory we decide for it. (Part I)
They tell him it was just “irritated” by some leftover memory, Beatty says does not have any enemies there. They promise to have the Hound checked out. Montag is worried that the Hound knows that he is a book thief though.
Montag is scared. He comes up with a plan. He looks up Faber and gets a replacement book. Then he will bring the book to Beatty. Yet when he goes to the fire house, he notices something.
The Mechanical Hound was gone. Its kennel was empty and the firehouse stood all about in plaster silence … and Montag came in through the silence and touched the brass pole and slid up in the dark air, looking back at the deserted kennel, his heart beating, pausing, beating. (Part II)
This is not a good sign. Beatty was waiting, but pretending he was not waiting. He immediately starts in on Montag, inviting him to a game of cards but really just wanting to make him sweat. Soon, they get a call that a tip has been made, and it is Montag’s house. He has been betrayed—by his wife.
Montag's earlier fear and disturbance with the Hound are foreshadowing for when he gets turned in, as well as evidence of a guilty conscience. When Montag's wife gives him up, he has no choice but to run. He turns his flame-thrower on Beatty, a very symbolic move, and kisses his old life good-bye.