The answer to this question can be found at the very, very end of "The Sieve and the Sand." Throughout this entire section, Montag has been in full-fledged confusion and tumult, questioning everything that he previously thought was true. He realizes that he is unhappy, that his world is incredibly messed up, and that somehow, books hold the answers. He has started openly reading books. He has gone to Faber's house and planned an insurrection of the most insidious kind. He has freaked out in front of Captain Beatty. He started quoting poetry and yelling at Mildred's friends. So, to say the least, his behavior has been a bit odd, and even rebellious.
So, when he does to work and there is an alarm, Montag doesn't actually see the address on the report, but when they pull up in front of the house, he is astounded. He realizes, "Why,...we've stopped in front of my house." It is his own house that they have been called in to burn. His actions have finally been noticed and called in. The next section will explain who called in the alarm, and just exactly what happens at his house. I hope that helped; good luck!