There was an interesting "unintentional" experiment posted here a couple summers back. A fellow had left a proof set on the dashboard of his car in the Texas summer sun. The plastic was warped and looked like it had been in a fire. And all the coin obverses were lovely rainbow toned. The reverses still appeared newly minted. So I would say, yes that is entirely possible.
Absolutely possible. Left face up in a vehicle in the cup holder or junk portion of the console. Even in an album in the right conditions. On a window sill.
