At the end of section 2, Montag has returned to the fire station after reading poetry to Mildred's friends, against Faber's advice. While Montag thinks he has accomplished an act of rebellion, Captain Beatty knows all about his actions. When an alarm sounds, they race to the fire engine, only to arrive at Montag's house. So, Montag's unknown destination is his own home.
It turns out that Mildred has reported Montag, effectively and permanently severing their already irreparably damaged relationship. This deepens Montag on the path to isolation/alienation, and indirectly influences his turning the hose on Beatty and the other firefighters. It also highlights Montag's individual struggle against an oppressive society, that wants nothing more than apathy from its citizens. In a sense, this scene is the point of no return for Montag. Certainly after killing Beatty he can no longer remain in the city. He is forced into the wilderness with others who revolted.