In Yosemite what does the indicator that looks like a scroll bar mean at the bottom of the desktop? I've noticed that at the bottom of my desktop Yosemite has started displaying a strange sort of indicator - it looks like a horizontal scroll bar, it's a thin grey rectangle with rounded ends.

If I use large dock icons it overlaps with the dock itself, if I shrink the dock you can see that it is unrelated to the dock.

And no, it's not part of my wallpaper!
It looks like a bug, not some sort of meaningful indicator as I first thought.

After quitting every application and restarting it's now gone away!