Ask HN: How is Google policing screenshots of Google Maps?

I received a notice from Google regarding usage of Google maps without attribution. Which I ignored at first because we only use Google maps on our internal password protected site and we show all attribution. After another threat from Google that they would block our usage of the maps API if we didn't comply, I did some searching around and some back and forth emails and found that they were referring to a screenshot taken of some streets in Google maps that was being used as a background image behind our contact form at the bottom of one of our pages.

So my question is, how is Google discovering usages of their images in this case? Are they using some type of steganography? It seems that they would need to automatically process far too many images for it to be worth the effort.