Righto, it was a glitch.
When I was setting up my dual monitor palettes panels to mimic my CS5 ones*, I unchecked Window > Application Bar. Upon a restart of my Mac I rechecked Window > Application Bar and it appeared in the exact same place as the artificat. Unchecking again removed it for this session, across relaunch, and across restarts of the system.
Chalk it down to teething problems
Ok, now I'm documenting in case others have the same issue.
The artifact reappeared after a few relaunches. I think I've got rid of it permanently by:
1. Switching on Application Bar visible, i.e., Window > Application Bar (make sure it's checked)
2. Docking the Application Bar to either the top or bottom of my screen, so it's not floating around in it's own loose panel
3. Switching off Application Bar visible, i.e., Window > Application Bar (make sure it's unchecked)
So far so good.