There have been incidents that Google didn't acknowledge at all. There was one day where all sites using custom domains were redirected to Google.com for several hours, including their own Chrome Web Store. (which, I presume, runs on App Engine)

It eventually resolved after a few hours, but both during & after the incident, there wasn't any public communication from Google.

https://groups.google.com/forum/#!topic/google-appengine/IIv...

https://groups.google.com/forum/#!topic/google-appengine/voR...