I have a 97 Passat GLX and my sunroof just exploded on me today. It was also about 25F last night and was about 40F today when it happened, although it was very windy in my case, and I had the sliding partition opened to let in more light. I had been driving for 10 minutes with the heat on pretty high to reach the kids in the back seat, and it exploded with a loud pop and the glass expanded outward slightly and remained in that position in a million little fragments.
The VW dealership guy said "in 15 years here, I have never heard of that happening, something probably hit it" which I guess could have been possible, but it seems like it would have been dented inward toward the cabin were that the case. Unfortunately it's probably impossible to prove that it exploded spontaneously, and I have a $500 deductible which keeps my rates low, but this might just cancel out that effect as the repair will be $550 unless I can locate the glass cheaper and have it installed by a guy I know.
Please respond if you have had a similar experience.