In The Egypt Game, why is April at first concerned about the empty mailbox and then later concerned about Dorothea's latest letter?

With a
crazed killer on the loose in the neighborhood, the kids are in lockdown. That means no going
outside to play; specifically, it means no Egypt Game, at least for the time being.

As one can imagine, it's difficult for the kids being cooped up inside all day. It's
especially hard...