Comment 3 for bug 840821

I noticed this by accident a little earlier today and looked into it a bit. Originally I thought it was a bug, but after looking into it a little bit I think its just a feature.

When you start a Unity desktop, Nautilus gets started with the -n parameter to manage the desktop. This "feature" allows you do do a quick search for desktop items. This is where the little search box comes from. If you kill nautilus then this search bar will not come up any more.

I'm adding a screenshot to demonstrate how it works. If you don't have any files or folders on your desktop - add a few to play around with it. Then, click on the background(make sure "Desktop" is in focus) and start typing. It does a quick search on any of the files/folders that you have on your desktop.