Exactly - the defect didn't show up until it was hit with a small amount of heat. That is why you didn't notice it before. It is a manufacturing flaw. Looks like the thin film is separating from the plastic backing.
A little CD trivia - the top side of a CD is much more vulnerable to irreparable damage than the bottom side that the laser shines through. Most people don't know that. The plastic on the top is much thinner.
-S
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