Thank you for your response. The top of the capsule was actually removed. I just left it laying inside of it to take the picture. It is visible from all angles. Here are some close up photos that may help.
I inherited them so I am not 100% sure but I seem to recall her having them for a very long time. It doesn't answer your question but I do know they have been in their covers for at least the past 25-30 years.
It's difficult for me to discern from your images - they are fine, you did a good job with them - which of the two scenarios I can imagine could have caused the issue actually did so.
Scenario 1: As already mentioned by carlp007, a liquid could have dripped on to the coin and caused the discoloration.
Scenario 2. Something could have dripped on to the proof die at the Mint prior to the coin being struck and damaged the polished finish of the die.
In either example, I would imagine the liquid in question was at least mildly acidic.
Multiple examples of the same issue would point to a die-related issue; a single example points to an event specific to the coin. In the absence of others here reporting the same discoloration, I would side with the view that a liquid dripped on to your coin when its capsule was off and the liquid caused the issue.
In any case, I wouldn't consider it an error by the definition for the term that is typically used for coins. Also, I don't believe it is something for which collectors will pay a premium. In fact, it would likely be the opposite.
Collecting history one coin or medal at a time! (c) commems. All rights reserved.
I don't think that it would necessarily have been opened, although that is a possibility. Don't forget that these are hand packaged in a different location after they have been struck so it is possible something could have happened between then. It doesn't really matter though as it is still damage