My Cat's Eyes Roll Back In His Head, What Causes That?

You are seeing your cat's third eyelids.  These eyelids are located at the edge of each eye by their nose and elevate from the bottom corner of the eye.  They can elevate normally when relaxed or sleeping. 
When the third eyelids are elevated during the day when your cat is awake and alert this can signal illness. 
A small amount of brown or white discharge is normal especially in the morning--the eyes are getting rid of dirt/debris.  Many people awake with "sleep" or a small amount of discharge in the corner of the eyes.  If the discharge is green or yellow in color, if your cat rubs/scratches his eyes, or if there is redness around the eyes your cat needs to be examined.
If you have any concerns it is always better to have your cat examined by a veterinarian to make sure what you seeing with the ocular discharge and third eyelid is normal.

I am out of town and just heard from the pet sitter that my cat's eye is missing or rolled back.  No other injuries so not a dog attack.

What can I/she do?
What causes this?
What is the treatment?

You need to see a vet about this, your cat could have a problem with blood reaching its brain, which could lead to epilepsy.

Does she/he have any other symptoms? Is it a kitten or grown cat.

Eyes rolled back (where you can see a second eye lid) means a deep sleep like as in sedated sleep. My cat showed that when given a sedative at home.

It's not normal in a healthy cat.