getcooking - 4:39 pm on Apr 25, 2013 (gmt 0)
This is interesting. About 2 weeks ago I blocked the user agent "AppEngine-Google" because I found a proxy had scraped one of our pages and it was in the "omitted" results when doing a routine string search for text on that page. A little more digging and I found more of these and was able to trace it back to that user agent.
Now, two weeks later, the traffic on our Panda-hit site is surprisingly up by 10%. I'm not sure if there is a correlation or not, but given what you found now I'm wondering if that has been affecting us. The site in question has had real problems with ranking the homepage. It ranks on our brand name and that is about it.