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!

Mystery grey rectangular icon on Yosemite desktop


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!