Around here it has to be badly placed sprinklers. After that it's windows that set up under large overhangs so that the minerals get only slightly wet or damp and then dry again making the dirt harden into stains over time.
Here's a couple pics of a job we did a while back where sprinklers had made a mess of the windows.
https://www.facebook.com/photo.php?f...type=3&theater
https://www.facebook.com/photo.php?f...type=3&theater