What does Mildred believe happened to Clarisse?

When Montag hasn't encountered Clarisse for a few days, he asks Mildred if she's happened to see her. Mildred says no. However, she also says that Clarisse's family has moved away and that she believes Clarisse is dead. Mildred states that she thinks Clarisse was run over by a car.
When Montag presses Mildred as to whether she knows for a fact that Clarisse is dead, Mildred says she isn't sure. She is certain the family moved but is only "pretty sure" that Clarisse is dead.
While we don't know that Clarisse has actually been run over by a car, it makes sense, given that Clarisse is given to taking walks, a very odd habit in this particular culture. Beatty tells Montag that Clarisse is better off dead because she was such an oddity, but Montag is upset by her death.