I have a windshield crack on a new vehicle that I originally thought was a stress crack, since it appears to originate at the edge and move toward the center, and I did not ever notice any impacts occur. However, when looking closely I see at one point in the "fault line" a pattern that could maybe be argued is an impact site. I guess the real question is: could something like this develop in the middle of a stress crack, or does the existence of this pattern mean it is undeniably an impact crack? I realize photos can only convey so much, so hopefully they are helpful. I took two photos, in the first one the windshield is still dirty, but I think the crack is focused better there. Thanks!
I cannot tell you with 100% surety this was caused by an impact, but I'd suggest it probably was. Glass is a strange beast. It does have some flexibility to it, but if you get a stress riser in it, a crack can form. A rock impact creates a HUGE stress riser in the glass and the crack runs from there.
If you'd like to know more about stress risers, you can read this Q&A I wrote.