Mystery Stain

My husband has been traveling and he just got back.  I go to treat the ring around the collar and any other stains on his shirts with My Clean Shirt.   To my surprise, one of his shirts has stains on the upper back area and down his sleeve.  It almost looks like coffee stains and like someone threw a cup of coffee at his backside.  I asked my husband about the stains and he has no idea what it is or how it got there.  I proceed to treat it with My Clean Shirt and I will be letting it set for 24 hours.  Stay tuned...