FWIW, I found in a box in my garage (damp, hot in summer, cold in winter) a Kodak disposable camera (expired 2003) containing a half-finished 400 speed film. I recently finished off the film and sent it away for cheapy D&P.
The "old" shots had faded as if badly underexposed, while the recent shots were as good as fresh film. But, oddly, all the edge markings were bold as a fresh film! How do we explain that?