I have been using Invisalign Retainers for about a year now, and I found these "brown spots" on my retainers around my "molar" areas of the retainer. I have tried brushing my retainers with toothpaste, and letting my retainers soak in a normal store bought "denture cleaner" but these brown spots haven't gone away. I do believe it may be some sort of food, but I don't know for sure. It may also be something decaying.. I really want to get rid of them. Please Help?