I have one of those medium-size travel suitcases in my room at present and it seems to be a magnet for mildew. I don’t really get it as I leave it out in the middle of the room (ie it’s not stored in a dark cupboard), and the room gets a good airing every day. But give it a week or two and there’s mildew/light mould all around the outside of it. What gives? What’s the science behind this?