Same happened with my old civic, one day while driving the sun was shining and something caught my eye, it was a 2 inch crack, just like you I never saw/heard anything hit the window on the day it appeared or any time before the day. After a few months it grew to about 6-7 inches! Shortly after I sold the car so I never had to deal it in the end, and glad I didn't because it was a screen with a top tint in it so wouldn't have been cheap