Seeing Things That Are Not 'Real'...
 

https://www.yahoo.com/tech/s/ten-minutes-eye-contact-leads-204504440.html

When I was young, I would stare into a mirror or at something for an extended period of time and would ‘see’ things. I thought I was the only one and frankly figured I was nuts! Now at night, when I close my eyes, before I fall asleep I can imaging shapes and I can actually see them on my eye lids! Maybe I am cra cra…

After my car accident I had mysterious shapes floating around in my eyes that I could effect by blinking.......eye floaters, not monster shaped though.

What you may be seeing is the transition between being awake and being asleep.

I have done that. I never told anyone for fear they would think I was crazy
I wonder what causes it. I boxed ten years it could be from a head injury

That can be a sign of a retinal tear...not uncommon after car accidents.

Ptsd

I think this is relatively normal/well understood. You brain only processes a small amount of the information that the eye receives and it fills in the rest based on some sort of "from experience, this is what I'm expecting to be here so I'll sort of make stuff up at this point". The vast majority of the time, this works perfectly fine. If you sit in a dark room and stare for a length of time (or some of these other situations), your brain gets bored and starts making up weird stuff. Like this:

An Optical Illusion that Explains the Origins of Imaginary Monsters

This phenomena also plays out in other ways (like the famous "guy in the gorilla suit walking through the group of people tossing a basketball back and forth"). The brain isn't expecting to see a guy in a gorilla suit, so it just doesn't process that information.

I had a experience just like this once. I was at an intersection getting ready to turn left out onto a highway (I was leaving Barber Motorsports Park, interestingly enough). I looked to my left, didn't see anything, looked to my right and didn't see anything and glanced back to my left as I started to pull out. When I glanced back to my left, there was a guy on a motorcycle right there on the road. I would have sworn in a court of law that that guy wasn't there 2 seconds earlier, but he was - my brain just didn't process it.

I used to work at a photolab and I was the guy in a DARK room operating the film processor. There is no safe light for color film and the darker the room the better. It was not just kinda dark, it was deep in a coal mine dark. I would often be in there for an hour or so and it was like being blind for an hour. More than a few times I "saw" something move when I was alone in a room with the doors shut. It was just my brain trying to see something. When I walked back out into just normal room light I had to squint because it was like walking outside onto full sunlight with new white concrete on the ground.

I would get all the film ready to put on the machine faster then the processor was capable of accepting so I had to just pace back and forth in a room with no light at all. There is nothing else to do when in 100% dark but think and pace. And yes, I walked into the wall more than a few times.

I actually DO have PTSD