Well, all I can say is, does this look like normal "toning" on a 2-year-young coin fully sealed in mint packaging..?
http://forums.silverstackers.com/topic-58272-rwanda-african-rhino-toning.html
Not saying this is going to happen to them all.. But I'm not willing to take that chance. Because given enough time & the right conditions, I'd say that plastic is a death-trap.
Mine immediately go into air-tites -- after a quick dip to remove any residue, followed by a distilled water rinse & gentle pat with microfiber cloth (and then air-dry).
As my observations lead to PVC. Could be wrong.. but doubt it.