The Most Obvious Symbol award goes to the movie The Sacrifice, which is mentioned a couple times in the book. Early on, Nora goes to see the movie, "the latest urban chiller" (3.79), with Vee to do a review of it for the eZine. Vee summarizes the premise: "Can you imagine? […] Living your whole life never having a clue that the only reason you're being kept alive is to be used as a sacrifice?" (3.82) Oh, the irony in that question.
Throughout the story, Nora uncovers many things affecting her life that she never knew about before, including the fact that Patch wants to sacrifice her to get a human body. Adding to that is the dramatic moment when Nora contemplates sacrificing herself for Patch rather than having Jules shoot her.
The movie's blunt title is a major clue to the general arc of the book. Hush, Hush's plot revolves around secret manipulations and plans for sacrifice, and early mention of a similar plot in The Sacrifice frames expectations for an equally creepy storyline as we read.