Fred and George probably joined the Order of the Phoenix and learned of my place, or they are smarter than I gave them credit for and they figured it out on their own. I tie up the little bag and drop it into my illegally expanded schoolbag. A few nights I spend some time learning how to change the bag into different styles without disrupting the content. I also added some extra items in it, like a long black cloak, extra set of clothes and even a survival kit.