It's an eye! Probably... from the pigmentation and the shape and location, it probably has to be. However, it doesn't seem to be growing or changing. Eyes were complicated things, and it would take some time for this little bleb to be full-sized and functional/

But once the speculum is removed, the orbital fat that had formerly been cushioning the eye fills in the socket and hides the little eye from view.

Oh well. At least it was proof that Rin's theory was correct. If the eyes would be visual or if Lilith's curse would persist was yet to be seen...

Though, after the pinch of the second anesthesia, Rin's breathing gets deep and even. It seems like he really did fall asleep.