Rear Window Wiper Washer problems of the 2003 Ford Windstar

I have roughly a 40 minute drive to work. It was a cold day and drove with the heat on and the roads were fine after a snow the night before, but there was no snow left on the vehicle. I came out at the end of the day to find the rear window missing about six-eight square inches and shards of glass below in the snow. The glass appeared to have been hit from the inside, but I think it exploded from within (nothing else had been broken or missing from the inside). I'd had no trouble with the rear defogger or wiper. I found out that a CO-worker noticed it at lunch, so sometime between 8:00 am and 1:00 pm this happened. No one saw anything.