Is there warranty protection for cracked windshield which IS NOT the result of a rock or other debris? the crack started underneath the bottom seal on the windshield; there is no evidence of a rock having hit the windsheild and caused the crack.

 

I doubt Lexus is going to cover your windshield.
I bet there is a very tiny impact you cannot detect because it is now in the crack.
Check your insurance coverage as windshields are usually treated differently with a lower deductible due to their lower costs and frequent need for replacement. Just tell them it has a crack and you think something must have hit it but don't recall. They will probably cover it and send out one of their approved glass guys to do the job in your driveway, not further questions asked. It is a non fault coverage and will not affect your rates. In winter with more potholes etc there are a lot more pebbles and road debris being thrown around.
I even see them doing windshield replacements in the side street next to my office, twice last week in fact!

 

Bummer re the windshield! The way the crack developed sounds similar to this existing thread:

https://www.clublexus.com/forums/rx-...-rx-350-a.html

Makes me wonder if the odd shape of the glass has anything to do with a higher chance of failing.