This reply discusses a slightly different situation to "after image", and concerns early training for "see and be seen" procedures in visual flight rules aviation. The effect you have noticed is well known to pilots, especially combat pilots, and the solution is to constantly scan the skies around you to identify other aircraft. It is astonishing how quickly a spot against a clear blue background can disappear if you stare at it, then to re-appear as soon as you blink or look away for a moment. It is possible that the human eye evolved to automatically detect movement and focus on that movement because in earlier times movement probably meant trouble. Also the eyes of plenty of prey animals probably behave the same way, and cats know it. If you've ever watched a cat stalking its dinner you'll see what I mean. The idea of scanning the sky while flying is to fool the eye into thinking something is moving, and therefore make it stand out, by moving the eye instead. Actually the exercise can be of benefit while driving or boating as well, in fact any good driving instructor will encourage you to have your eyes constantly roving the view in front, to assist in identifying smaller dangers such as bicyles, motorcycles, people, debris on the road and even parked cars.
So, what you have noticed is common to all humans and a great many animals, is quite normal and I threw in a few hints to overcome what could be a disadvantage if the phenomenon has not been recognised.
