What does Montag imagine to be outside his window?nope.
In the first section of the story, "The Hearth and the Salamander", Montag imagines, one night, what would happen if he opened the window because he thinks he hears a slight sound and sees a vague green luminescence outside his window. He doesn't open the window because he fears that what is outside the window is the Mechanical Hound. Montag has books hidden in his house and has had them from the beginning of the story. We know this because of the references to the ventilator grille mentioned as early as the first time the reader follows Montag into his house. Montag knows that the Hound somehow seems to suspect him and he knows what would happen if he were caught with books. The Mechanical Hound, along with Montag's paranoia, help to further the theme of alienation and loneliness. A dog is typically looked upon as a warm, loving creature devoted to man, whereas the Mechanical Hound is cold and devoid of all good qualities of dogs. Montag's paranoid feelings increase as the story progresses because he senses that Beatty is on to him, especially after the old woman lights herself on fire to burn with her house rather than give up her books.