netmeg - 2:00 pm on May 2, 2012 (gmt 0)
The page it hit on my site (home page) was ranking on the first page, but not #1 for many terms. As far as I can tell, the rankings never changed. But the site is seasonal, and it was off season, so it wasn't getting much traffic anyway. That's why it stood out so much.
I wondered if this was some kind of test, and if so, did I pass, fail, or what?
Someone on the post on my site responded that they had actually discussed this with an AdSense account rep and were told that AdSense was seeing a lot of this and they were mystified too.
I'm still getting around 20 or 30 visits a day by this. So it hasn't totally gone away.