Could anyone explain an unusual icon on my taskbar?

Earlier today, I restarted my computer. When I logged back on, I found that a new icon had appeared on my taskbar (where it remains); it's a small, grey-black cube with the letters "Fn" written in white, designed almost like a cell on the periodic table. I have clicked it repeatedly and it does not respond; likewise, nothing happens when I run the cursor over it. So far as I can tell, it signifies no process currently being run on my computer. Any ideas what it could be?