I've seen a few 'phanton scratches' over the years. In fact, I had one just the other day going down my arm. Strange thing was that it didn't hurt at all. The next day, I was holding my cat and when she jumped off, her back claws created a similar scratch on the other arm in approx. the same place. But this one hurt. Making the other scratchg even odder.
Now I don't have any strange noises or anything else paranormalish going on in this house. My family on my moms side can 'see ghosts.' I thing I inherited this because in certain places, I can see figures moving out of the corner of my eye. Haven't had such a thing happen to me at this house. Every ARMY barracks I lived in I saw these figures in some places around the building, especially my barraks in Germany that used to be an old WWII hospital…. So anyway, I really don't know where to pin this scratch. I might have done it to myself unaware. It's just strange because there wasn't any pain associated with it.
Now your scratch could be phantom, or you could have done it unaware. Best thing to do when you see a 'phantom scratch' is retrace/remember what you were doing from the time the scratch wasn't there to the time you spotted it. Another question: Is there anything strange that goes on in your house/room that you can't explain? If nothing really extraordinary happens in your house, it might be safe to assume you did it to yourself somehow. If if happens again, especially if another scratch shows up in the same spot, then would be a good time to pay attention and look for potential causes.