XG-9 eyepiece dirt
Craig, if you have more than one lens, check to see if it is there in both lenses, then you know it is in your camera. If it is not on your mirror then don't fret. If the camera still takes good images then you may have to do without a clear view if you want to use it. I had an old XE-4, XD-4, don't remember, but it had dark lines in a round pattern in the viewfinder, but it was there regardless of lens and not on the mirror, I don't have the camera anymore, but my ex-roommate has made some wonderful images with it.