A mystery wound is any unexplained cut/bruise/scar/scab/injury about your body.
I woke up with a gash on my hand a couple weeks ago. It was from just near the base of my thumb up to just under the base of my index finger. No idea how it possibly could have gotten there without me noticing. Maybe I have some sort of nerve damage in my hands, because I never feel it when I get cut, I just notice that I’m bleeding a short while after.
Weirdest one, however, is the little scar on my face. No idea what that one’s from, and you’d think I’d remember that one.