When the housekeeper throws away the matchbook, there are two references the the fact that this is he MacGuffin for the episode; another housekeeper says "Goodnight Mrs McGuffin" to her as she misses the MacGuffin, and then on the work board in the incinerator room, the sign showing "Mac Duff" as "In" has a tear on the "D" to make it resemble a "G", once again reading "Mac Guff In"