The mystery of the ventilator grille is something that Bradbury sets up initially and then seems to forget; it is alluded to only a few times before Montag finally reveals the secret: that he has been hiding books for years, and has a sizable collection of illegal contraband. This serves as foreshadowing for his slow move to individuality; even before he knew it consciously, he was rebelling against society. However, this is not revealed at first, but is hinted at during scenes where Montag is starting to question the rules of society.
"Any man's insane who thinks he can fool the Government and us."
"I've tried to imagine," said Montag, "just how it would feel. I mean to have firemen burn our houses and our books."
"You got some?"
"No." But in his mind, a cool wind started up and blew out of the ventilator grille at home, softly, softly, chilling his face.
(Bradbury, Fahrenheit 451, Google Books)
Before his reveal, when Montag takes out the books, there is little information about what the secret might be; however, since the entire book is about books being illegal, it should be fairly obvious that the mystery involves them in some way. He could have been hiding writing equipment, but that is not illegal; he could have been hiding drugs, but aside from Mildred's sleeping pill addiction there isn't any drug use. A person reading the book and paying attention to the clues in the text should be able to easily solve that mystery.