Loose rivets; no, I am not 100% sure about either of the first two occasions. The first time was several months ago and as it was a temporary localised event, I assumed that I'd perhaps inadvertently caught a glimpse of a reflection of the sun. As far as I can remember it was just the right eye and also the right side of my field of vision in that eye, but I can't be certain. Yes, I was alarmed.
The second event was maybe six weeks ago and I really can't remember too much about it except that I think it was left eye and the symptoms were the same.
The slight sensation in my eye; well, that's about as much as I can say. No pain or discomfort. Sitting with my eyes closed I was "aware" of my right eye in a way that I am not normally (or now).
As far as I know, it's a normal optician I'm seeing but I can check on that.
After the original response from one dot right, I have read up on migraine (which I've never had before) and it does seem to tick lots of boxes.