I work for a windscreen company and it is common for people not to notice very small chips. In the line of the crack somewhere usually towards the edge of the glass as that is the weakest part, then from heaters blowing on to a cold or frozen screen it goes. If there is definatly no chip (wont even be able to feel it!) and it comes from the bottom from under the scuttle then it could have gone from the hight block it sits on on the impact and slowly moved up! dont know if that is any help at all.