What did Montag imagine being outside his window?
Montag imagines that the Mechanical Hound is outside the window.
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 Hound, he thought. It's out there tonight.
The Mechanical Hound is a travesty of technological progress, created to destroy and to kill instead of to save. In this dystopian society, the fireman's hound does not sniff out the wounded and weak in order to save, it sniffs out the innocent and terrified in order to kill. It is an eight-legged metal beast with a "four inch hollow steel needle plunged down from the proboscis...to inject massive jolts of morphine or procaine."
The Mechanical Hound's internal computer can hold any combination of someone's chemical makeup. While Captain Beatty asserts that the Hound is only "copper wire, storage batteries and electricity," Montag suspects that someone has programmed the Hound to recognize his own chemical make-up: "so many amino acids, so much sulphur, so much butterfat and alkaline." He tells Captain Beatty that it would be an easy thing for anyone to program this information into the Hound, as all the firemen's chemical percentages are recorded in a master file downstairs in the fire station.
Later, in his own home, he still feels uneasy that the Hound is targeting him. He fears that it is just outside the window of his bedroom; because of this fear, he keeps the window closed.