I have experienced this too, but in the end, it turned out that I did not close my eyes all the way, and that I was seeing through that small opening. Very small, but enough to see. If I close my eyes very firmly, I see nothing.
Try to block your eyes with something (but try to not block your 3rd eye, just two eyes). If you keep seeing with eyes properly closed, then you might be very open, and your third eye might be giving you an image of your surrounding.
Third eye deals with astral sight, not spiritual, that's why I am thinking it is less likely that it is the source of your vision. Although, I might be wrong. Try to test it.