Sirius Black: Brilliant, Snape. Once again you've put your keen and penetrating mind to the task and as usual come to the wrong conclusion.
When Hermione and Harry go back in time, they duck and hide from Malfoy (after being punched) and his friends as they leave. The rectangular outline of a power pack (battery pack for hidden cordless mic) is visible at her lower back, under her pink jacket. It is visible in a few shots, including when they are at the pumpkin patch. The outline of Harry's power pack at his back is visible as well, as he bows to Buckbeak at the pumpkin patch, when they rescue him.
In order to acquaint himself with his three lead actors, director Alfonso Cuaron had each of them write an essay about their characters, from a first-person point of view. Emma Watson, in true Hermione fashion, went a little overboard and wrote an 16-page essay, Daniel Radcliffe wrote a simple one page paper, and Rupert Grint never even turned his in, as he said that is what Ron would have done.