Is it one patch about mid way? If so it's likely old blood from a woman's paraphernalia failing. I often see this. Whatever you try use something like a q-tip or small brush so as not to spread it. Try dampening the middle of the stain with a few drops of water then applying corn starch or baking soda. Hopefully the water will reactivate the stain a bit and the starch/soda will absorb it. You could also try a peroxide/water 50/50 and let it bubble then apply the powder. If you get some improvement in the middle of the stain then gradually work to the outer edges. When you've applied whichever powder allow it to dry overnight then vacuum off the residue.