what is waiting outside montag's door? and why?

The Mechanical hound is waiting outside Montag's door. It is used to track down people on the run. It has a needle that injects morphine into the people. Montag is, of course, intimidated by it, that is why it is outside his door. It is an ominous sign for Montag.

It is implied that it is the The Hound: Montag thinks as much, and the 'faint drift of greenish luminescent smoke' may be linked to the earlier 'green-blue neon light flickering in its suddenly activated eye-bulbs'. Early in 'The Sand and the Sieve' after they hear a scratching at the door Mildred asserts it is 'only a dog' and there is 'the smell of blue electricity blowing under the locked door.' When The Hound appears after Montag has killed Beatty it is preceded by the phrase, 'The blowing of a single autumn leaf'; this links with the ealier phrase, 'the motion of a single huge October leaf blowing across the lawn and away.'

Beatty, of course, also waits outside Montag's door, but the question 'what is waiting' seems to suggest that 'The Hound' is the required answer. However, it is noteworthy that The Hound's aggressive reaction towards Montag is followed promptly by Beatty's explantion of its functioning; that Beatty's visit to Montag's home is followed by The Hound sniffing at the door; and that Beatty's death is followed immediately by the arrival and destruction of The Hound: the two are structurally linked and this suggests that Beatty, as a representative of authority, and suspicious of Montag, is in command of The Hound.

It is confirmed, retrospectively, that one function of The Hound is to track and eliminate dissenters, criminals, runaways, free-thinkers, anyone considered to be a threat to society. We are told that The Hound operates via a super-sensitive mechanical olfactory system that can be programmed into it and that the chemical makeup of each fireman is kept on record in the firehouse. Montag displays paranoia in suggesting that someone has altered his data to make The Hound dislike him. Beatty recounts the suicide of a fireman at another station who altered his own data so The Hound will exterminate him.

However, we are invited to consider that there is indeed an actual change in Montag's spoor, subtle changes in his chemical make-up. Clarisse's appearance in the novel is signalled by Montag's subtle sensory perceptions, ' Perhaps his nose detected a faint perfume, perhaps (his) skin ... felt the temperature rise". In her presence there is the faintest impossible breath of fresh apricots and strawberries in the air. This is the same basis on which The Hound is said to operate. Simply breathing in Clarisse's essence, or hormonal changes resulting from their interactions, could well have altered Montag's chemical makeup to an extent that could be detected by The Hound.

Montag states that it's not the first time The Hound has threatened him, 'Last month it happened twice', and of course his physical contact with the books he has been secretly reading may linger, or changes in his thought patterns or emotional responses as a result of reading may be detectable.

It is made clear that The Hound doesn't operate independently but is the servant of those in authority; nowhere is this more obvious than when it is used to dispatch the innocent man out walking who, on olfactory evidence alone, could have been easily distinguished from Montag.