There is a hidden reference to the seventh book during the time at Grimauld Place during Christmas. When you are helping Ginny find Kreacher's items and you pull off the painting from the wall, notice the locket hanging from the wall. Ginny says it will not open. This is one of the Horecrux that is mentioned in the sixth and seventh books. It is R.A.B.'s Locket, which he stole from Voldemort.