Must be an unfinished or deleted button. The image is standard `NSAdvanced` system image, which is usually first in the drop-down lists in Xcode. When a developer does not have final graphics and needs to pick just anything they often click the first one in the list, this one.
It's not a button, and isn't supposed to be one. It even has a shadow underneath it - I'm hoping it's just a way of quickly being able to identify that a Mac is running a beta of Mac OS X, and that it will go away with the official release.
