We also have seen this phenomenon. The first time was with bags that came from Europe with a variety of materials housed in them. But, and this is worrisome, we have also discarded old bags that were never used because they had yellowed considerably. The bags were stored in powder coated steel drawers for a few years. Due to needing to replace the drawers they were then placed in commercial plastic drawer units for temporary housing. To add to the confusion as to why, not all the bags stored this way yellowed.
