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.

Go into toolbar customization mode, and drag it out to get rid of it.

This only seems to happen when you install Yosemite by upgrading from the previous OS X. It's not there on a clean install.

I may be wrong though. When I first installed Yosemite by upgrading, this icon was there. This morning I did a clean install and it's not there. Maybe it's an update that fixed this? I don't know.

That's likely the Dropbox Finder item. This is where it appears by default when installing Dropbox. I'd guess that Dropbox Finder assets aren't completely compatible with Yosemite yet.

I'm running Yosemite and I don't see that, you might want to try updating to the latest version.