Mildred is horrified by the thought of Beatty's visit in Ray Bradbury's Fahrenheit 451. What does she stand to lose?
Right before Part Two, The Sieve and the Sand, starts, Montag's and Mildred's front door alerts them, saying that someone is at the door. Neither one of them opens it, but Mildred is convinced that Captain Beatty has come back. "'He'll come in,' said Mildred, 'and burn us and the books!'"
At the beginning of Fahrenheit 451, Clarisse says that most people are scared of the firemen, implying that even the people who abide by the ban on books still possess an almost irrational fear of individuals like Montag. But perhaps their fear is actually very well founded. If, by some mistake or random chance, the firemen decide to visit someone who has never so much as touched a book, there would be no one to protect them. In the world of Fahrenheit 451, no one is going to fight for the rights of their neighbors or even of their close "friends." Mildred knows that being in the cross-hairs of the firemen essentially means that your life is over. No one will stand up for you or care what happens to you.
But Mildred knows that they are not innocent. They are in possession of books and she knows that it is only a matter of time before the firemen discover the hoard that her husband has built up. In the life that Mildred lives, no one helps each other. Hope is a rare commodity in her life, so instead she numbs herself with TV programs and sedatives. Mildred knows that the only things that get her through her life could be taken from her any second now that there are books in her home.
Mildred frequently calls the characters on TV her "family." She says that they "tell me things; I laugh, they laugh! And the colors!"
And then she goes on to say that if Beatty discovers the books then he might "come and burn the house and the 'family.'"
Mildred's life is very fragile and she knows it. She doesn't remember how she and Montag met and she doesn't really care. All Mildred cares about her is numb life and the "family" on TV. Montag's books represent a clear threat to her frail little life, which is why she eventually betrays her husband.
If Beatty finds out what Guy Montag has been up to (and if he chooses to make an issue of it), Mildred can lose everything that is important to her.
Usually, you would think that would mean that she would lose her husband and that is what would horrify her. But Montag really does not mean all that much to her. The real problem, for Millie, would be losing the parlour walls. If Guy is caught, the house will be burned down and there will go her "family."
So Millie is afraid of losing all the "people" that are important in her life.