The polycarbonate must be either light or moisture sensitive, becoming opaque after a certain amound of exposure time.

They could get the same effect by just shipping the DVDs in leather slipcases instead of card or plastic ones. I've got a "special edition" CD single of, what else, Tainted Love that came like that, and the destruction wrought on the metal layer (not the polycarbonate) by, I presume, tanning chemicals in the leather has to be seen to be believed. There are macroscopic, like 1-2mm, holes in the metal, even though the plastic is untouched and smooth. Where the metal isn't absent it's tarnished nearly black.

This disc came in jolly handy when testing Rio Central's resilience in the face of unreadable CDs...

Peter