There are a few reports in the Google Webmaster Help forums that Google is sending out webmaster alerts in Google Webmaster Tools without any detail.
The first report said:
Received this alert in my webtools this morning
"Alert: Google has noticed potential problems with this site. View »"
But when I click view there are no messages listed just a radio box with delete option nothing else. Also says 1 message in the left menu.
That post followed by two "me too" replies. I am not sure if this is a wide spread issue or not. I personally have no new alerts in my account, so it is hard to tell.
Forum discussion at Google Webmaster Help.