What did Mildred fail to tell Montag about Clarisse?

Montag questions Mildred about Clarisse, who he has not seen in days, and Mildred says she had forgotten to tell him that Clarisse was struck by a car and killed four days earlier. Her family has since moved away. Montag is very upset to hear this news and can't believe Millie forgot to tell him. He falls asleep with his stolen book hidden under his pillow.