well, I'm not into spiritual things and all, so I'll give you my "scientific" opinion.
I guess It could be a "floater" as discribed earlier. I don't know if it's the same thing as I have... I have little "veins" that sort of float in my eyes (eww) and I was worried about what I saw sometimes too. I went to the docors and he told me it was nothing to worry about. My eyes are just very sensitive and pick up the shadows of those little veins, so you see little transparant grey lines and blobs. I can even see blood running through them when it is a very bright day

Anyway, it's nothing to be worried about.
Another thing could be that it's your brain fooling you. When you are very tired, relaxed or concentrated, your brain begins to wander, causing you to see things. Sometimes you think that something moves (most of the times out of the corner of your eyes) Sometimes you see that a door is slightly open, and when you look again it's still closed, that sort of thing.
I think that might be the case, since it happens when you are at the computer with your headphones on. You are very concentrated or relaxed by the music which causes your brain to wander off
