What sign does Melanie say to look for?

When Elizabeth asks what sort of sign she should look for, Melanie answers that she (Elizabeth) should look for a secret omen.
In chapter 9, April tells the rest of the group (Elizabeth, Marshall, and Melanie) that they have been summoned back to Egypt. She mysteriously reports finding a feather on her window sill and concludes that it was a token from the mighty gods.
Meanwhile, Elizabeth is worried about breaking away from the trick-or-treating group; she doesn't believe that it's a good idea to visit Egypt on Halloween, of all nights. So, she is especially nervous when April makes the eerie announcement that the group has been summoned back to Egypt.
Elizabeth is doubly concerned when April also suggests that they should look for a sign to determine the validity of the summons. She then inquires about the kind of sign the children should look out for. At this point, Melanie chimes in to say that everyone should look out for a "secret omen." 
April concurs with Melanie's assessment but suggests that the sign need not necessarily be a pigeon feather (as Marshall suggests).