He can rule out vandalism, which leaves a big smash (the voice of
experience!)

The most common cause is the effect of temperature changes when there is a
rock pit in the glass. I moved to cold country about 5 years ago and learned
cold nights cause most of the cracks here. Our Toyota dealer made the
determination whether our windshield crack was a defect covered under
warranty or damage to report to our insurance by running a ballpoint pen
along the crack. It hung up in a pit, indicating it was damage that had
grown into a crack... no warranty. It ran from the bottom edge near the
driver to the left edge and appeared when we washed the car after it had
been sitting in the sun.

As TeGGeR often points out, it is important to be sure it was not rust of
the weld around the windshield that did it, because the next windshield will
suffer the same fate if the rust isn't repaired.

Mike

Our 04 RX330 had a stress fracture crack.  The dealer determined it
wasn't due to a rock chip, vandalism or anything like that and
replaced it under warranty.

So, if the crack is really smooth from the side of the windshield to
the end of the crack, there is a chance it could be a stress fracture.