In Fahrenheit 451, Mildred clearly discovers the book under her husband’s pillow when Fire Captain Beatty comes to the house when Montag doesn’t show up for work.
Bradbury has already established that she doesn’t feel much empathy for anyone — for the woman who burned to death with her books the night before (“‘She’s nothing to me; she shouldn’t have had books.'”) or for animals (“‘I always like to drive fast… It’s fun out in the country. You hit rabbits, sometimes you hit dogs'”).
So why doesn’t she pull the book out from under the pillow and turn in Montag immediately?