It looks like a piece of tape or something sticky "pulled off" the finish in that area. Home improvement centers sell "finish pens". These are just like stain pens, except they are loaded with polyurethane (99.99999% oil based). You are welcome to purchase one of these finish pens and use it to "fix" this bit of missing finish.

Always test things like this in an out of the way area (like in a closet or behind a door) just to make sure you won't be adding insult to injury (ie. making it worse).

Once you are happy with the test patch (leave it for about 1 week...just to be sure), go ahead and carefully apply the finish to this little area.