I'm gonna say grease filled die or small possibility of being polished off the actual die. If it was indeed polished off you would see raised lines in the area of the fs but its unlikely as many more probably would have turned up. Worth a look under magnification though you got me curious now.....
I've been skeptic about it myself, but have kept a watchful eye on it given all the talk about it online and sure enough, today I seem to have also found one as well. Not the best condition, but nonetheless, here's some pictures: