Yesterday I noticed long red scratches on my arm that I cannot explain, having no memory of recently being scratched, and no memory of my arm recently being itchy or irritated. Below is a photo taken about a day after I first noticed such scratches. Such scratches are particularly baffling when they appear in the winter, as I know I have been wearing a heavy jacket to go outdoors, which rules out any explanation such as brushing by a branch. I also wear long sleeves in the winter, which pretty much rules out any daytime appearance. I am never disturbed or concerned when I observe any such anomalies. I see a variety of attempted explanations on the Internet for this type of scratch, which include "mattress springs" and cats, neither of which I have.
Postscript: A few days after publishing this post, I found two other red scratches, on the back of my shoulder. As a test to see whether I had any sharp fingernails, I tried scratching the back of my hand vigorously, with the nails on both hands; but no red mark was produced.