I think it is getting data from the toolbar. I have seen administration pages show up in Google Webmaster Tools, even though there is no way for a spider to access to those pages due to password protection.
Originally Posted by bogart
Chrome is another distinct possibility, but I only use that to test web pages for proper rendering.
Google has a long history of gathering information without your knowledge or permission.
"Democracy is two wolves and a lamb voting on what to have for lunch. Liberty is a well-armed lamb contesting the vote." -- Benjamin Franklin