mogs
It's probably one of 3 things
1, The builders own labourer cleaning glass, with a chisel or sandpaper from what we've seen in the past!
2, Glass scratched during construction
3, Glass scratched by painters preparing frames
Using a WFP with (copious) amounts of water would not scratch glass. Your customer has probably never been able to see through her windows before!!!
Jake-Westclean