Wanting to save his books from Mildred, who is likely to burn them, Montag hides them outside. He goes to work, pretends to give up the "only" book he stole, but when he and Chief Beatty answer a fire-call they find all his books inside, scattered on the floor:
...their covers torn off and spilled out like swan-feathers, the incredible books that looked so silly and really not worth bothering with, for these were nothing but black type and yellowed paper, and ravelled binding.
Mildred, of course. She must have watched him hide the books in the garden and brought them back in.
(Bradbury, Fahrenheit 451, Google Books)
Mildred's obsession with conformity and living in the culture is so strong that she betrays Montag, giving him up for the chance to move and forget everything in the embrace of the television screens. Montag, feeling great anger, burns their beds and the TV parlor, trying to symbolically erase her from his life, furious that she is so blinded and so shallow. Her betrayal is the last straw, and he goes on to kill Beatty and escape the city before it is destroyed by bombs.
Montag thinks that Mildred saw him take the books outside and brought them in with her.