Montag has committed the worst crime in his society; he has taken and read some books. The irony of it all is he is a fireman--one whose job it is to burn books. After he sees a woman burn herself up for her books, Montag is physically and psychologically sick about his whole world. Before the woman burns herself, though, Montag steals a book from her hoard. Someone, probably his boss, Captain Beatty, must have seen him do it because that night after the woman dies, Montag goes home and hears the Mechanical Hound outside his home. He describes it as follows:
"But there was something else in the silence that he heard. It was like a breath exhaled upon the window. It was like a faint drift of greenish luminescent smoke, the motion of a single huge October leaf blowing across the lawn and away. The Hound, he thought. It's out there tonight. It's out there now. If I opened the window. . ." (48).
Beatty comes over the next day to find out why Montag isn't at work and lectures him on the history of the firemen. He "teaches" Montag the reason books became outlawed is for the benefit and happiness of humanity. He also says that firemen do get curious about what they are burning sometimes, and that's natural. However, if a fireman takes a book, he has 24 hours to burn it or the other firemen will.
Montag does not heed Beatty's warning and pulls his wife into his secret about the books he has. He makes her read them with him while he's trying to make up his mind whether to go back to work or not. Then, he and Mildred both hear the Hound outside their house as they are reading. They hear a faint scratching at the door. Mildred says, "It's only a dog, that's what! You want me to shoo him away?" (72).
Both instances of the Hound sniffing around Montag's house are examples of literary devices called Foreshadowing. Montag knows what the Hound does, but his wife doesn't. Montag doesn't seem to grasp the hint of the dog sniffing around, though, because he is so engrossed in his reading. Later, Beatty says the following:
"Old Montag wanted to fly near the sun and now that he's burnt his damn wings, he wonders why. Didn't I hint enough when I sent the Hound around your place?" (113).
Foreshadowing is a hint or suspenseful clue about something that will happen later in the story. The Mechanical Hound is sent twice from Beatty as a hint to Montag that he's in trouble; but, it is also a hint for the reader to provide a suspenseful foreshadowing of what comes later. Eventually, Montag is viciously hunted down by the Mechanical Hound, who seeks to kill him.