I've posted something like this before, but have you asked the customers if they have sprayed anything on their windows.
I had a job last year i think it was where the customer insisted they hadn't put anything on the windows, but for some reason they wouldn't come good like they normally did.
They admitted in the end that they had bought a spray from Betterware i think that was supposed to make your windows self-cleaning.
Obviously it didn't work but this left marks like you decribed.
Craig
ps. just seen the posts as i was posting this and i have heard nesting birds can cause this