What does Anna mean when she says that she lost her whistle 133 days ago in Prisoners? It just seems too specific coming from a child. And what does Mrs. Dover whisper to everyone right after it?
Sorry if this is a partial answer (I'll delete it if you think it's not up to standard), but for what it's worth, the script eludicates what Grace said:
ANNA: The emergency whistle Daddy gave me. I lost it... (thinking) Hundred and thirty-three days ago.
GRACE: She’s just like her father.
So Anna remembering the exact day is supposed to demonstrate her similarity to her father. However, I'm not quite sure how. Maybe because he's good with numbers as a carpenter? Has a good memory? Notes certain things others don't or can't let them go? (Then again, I don't remember much of this film, so maybe commenters can enlighten me on how this is also one of Keller's character traits.)
I found this answer on Yahoo:
Q 1. Why was there (coincidentally) a whistle down under the trans am? Did Anna drop it there "133 days" ago?
A 1. The girls were initially kept in that hole in the ground under the Trans-Am while the police searched the aunt's house for the girls. Anna must have dropped her pink whistle there. Her mother later bought her a new red whistle to replace the one lost. Joy and Anna had made it to Anna's house and Joy had found the first whistle so the girls were abducted on route back to the Birch's house.
My guess is that at some point, as most prisoners do, she began counting days in captivity.