What you’ve discovered there, is that the eyes look, but the brain sees. What you’ve described there is when your brain starts to ‘target select’ ( there is an official name for it, but I’ve forgotten what it is ). It’s basically the brain filtering out what it perceives to be erroneous information, because there’s only so much processing it can cope with. If you are approaching a junction ( for example ) and your relative speed to an object approaching from the side, is similar, effectively the object is stationary, so even though it’s in plain sight, your brain decides it’s no threat, so discards it. You effectively don’t see something in plain view. “It / they came out of nowhere” is the phrase that’s commonly heard, when this happens. Fighter pilots are trained to overcome this by constantly changing their head position, whilst engaging another aircraft. If you’re riding / driving, you can help it by deliberately altering your speed, or course, slightly, approaching junctions and / or roundabouts. That way, any approaching objects, aren’t effectively stationary, and you’ll be surprised how often something suddenly appears, which was always actually there. Look, look, and look again, near junctions and RABs.