Depends.
An object moving directly towards or away from you is much less perceivable, than an obejct moving sideways in relation to your line of sight.
This is because, like said in other threads about icons, over a certain distance human eyes detect movement with the help of visual cues - by measuring relative movement of a certain object in cotrast to the background.
Obviously, a movement towards, or away from you has no relative motion which human eyes can train on and judge upon, and only changes in the size of the said object.
....
However, what J_A_B says applies in other ways.
Detecting a movement which the observer was previously unaware of, works in the way I described above. However, once humans are aware of an object, the human consciousness focuses on the existence of the object, and applies different levels of "alertness" to the eyes.
Its how camouflage works in the natural world too. When there is a camouflaged animal hiding in the woods it is effectively, by all means, 'invisible'. However, once we realize that there is something hiding there, we begin to see the shape of the animal very clearly, no matter how well it is camouflaged. No amount of camouflage short of science-fiction light-bending technology, will hide a shape against its background perfectly. We literally(!) see things better when we know it is there - its the way how brain processes information.
Computer screens, lacking in detail of "visuals", make that process more difficult. The camouflage effect in the initial stages, aren't wrong... but in computers, when icons are off, a very frustrating thing happens - we see a plane out in distance, close up and engage. Then, the enemy maneuvers away, we flick the hat-key and... what the? It's not there! The unrealistic lack of depth perception, flares out in these occasions. (Thus, icons, are needed.)
