I have seen this in some previous version of ubuntu also, snapshots are showing a lucid desktop.
In screenshot.png, just below the mouse pointer, you can see a small bar just above the lower panel on the left side.
Screenshot-1.png shows this bar in the lower right hand corner, with the lower panel autohidden.
If I do a mouse hover-over the pointer changes to index-finger hand mouse pointer, leaving me to believe this is some kind of window.
Cant track it down, does not show up on 'ps', cannot close it, only solution is to logout or reboot.
Am quite worried this might be some hidden execution, pixel exploit, keylogging, etc.
Cannot always reproduce this, but it does show up every now and again. Anybody else seen this and willing to offer an explanation?
Give me a list of things to check on the next time I see this, would love to find out what process is at work here.