I have experienced the following a couple of times.
The first time in normal light conditions I was rubbing my eye and while I was doing it i started to see veins, Only with my eyes shut, and they were silvery bluish color.
The second time: I woke up this morning 5 am and when i was closing my eyes i kept seeing the same thing only for a second when my eyes were shut, but when i opend them it went away. This went away after a few minutes.
I want to make it clear that the veins I see are only with eyes shut, and they re silver in color with black background.
I have had my eyes checked couple of months ago also done optic nerve photo, pressure, evetything was fine. But I did not mention this because I did not experience it at the time.