Not necessarily a stone chip, but likely a tree branch chip that started it! This is the rear wing on my car so I feel like a stone hitting it is a little unlikely, but ultimately not impossible I suppose. The spot wasn’t growing at all when I originally noticed it, then one day it was massive. I always took care around it because I obviously didn’t want to make it worse by washing it.

I suppose it’s possible that the horrendous rain around here did it though!