I'm familiar with the user agent used by Google image proxy:
Mozilla/5.0 (Windows NT 5.1; rv:11.0) Gecko Firefox/11.0 (via ggpht.com GoogleImageProxy)

But it looks like Google is also loading images with a different user agent:
Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/42.0.2311.135 Safari/537.36 Edge/12.246 Mozilla/5.0

These are logged when Google receives the email rather than when the user opens the email.

Fortunately we do see another open using the usual user agent when the user actually opens the email.

Has anyone else seen these?