Split nictitating membrane?

Has anyone heard of an injury like this? My sister's cat (stray kitten I rescued) has a split nictitating membrane... There is a cut in the membrane, so it looks partly split. I don't know if she was born with it or it's from an injury, since she was a stray...
My sister mentioned it to the vet when she took the kitten to be checked and the vet checked the kitten's eyes (she had an eye infection) and the vet said they couldn't see anything there! I don't know how they could NOT see it, it's very obvious when you make the third eyelid visible, that there is a cut in it!
Anyway I am wondering if anyone's seen this type of thing before and if it could cause problems for the kitten, or what could have caused it.

How do you 'make the third eyelid visible'? Did you do this in the vet's office? You might want a second opinion.

Well, sometimes it's visible on its own but what I do is gently rub the eye and then open the eye, and usually you can see the third eyelid then...
I wasn't at the vet office or I would have made sure the vet looked at it...