Cracks on window

Hi I'm currently cleaning the traditional way and today I've noticed a couple of tiny cracks on my customers windows which I'm 99% sure wasn't there before I started. I've never had this problem and don't know what's causing it if it is me or the window ? I'm using cold water so know it's not that . Any help would be greatly appreciated 

Could it be rusty nails in the putty expanding and causing cracking ? 

It depends on what type of windows, old, new, wood, pvc, single pane glass, glass unit or south facing?

Can you verify please?