Update

The Volvo dealer went about repairing the windshield and seal, which requires removal of the windshield. The windshield was the original Volvo factory glass and it had small pitting from a stone strike (but no crack) in the upper middle section. We didn't know the pitting was there until dealer pointed it out. The dealer warned that because of the pitting, the glass could crack when reinstalling.

Sure enough, the glass did crack and a quality aftermarket replacement windshield was purchased and installed.

It rained last night, no leaks around the windshield, so we'll chalk it up as repaired. No one had an explanation about why the windshield seal came undone after six years, other than "it happens sometimes."