I am a pilot and have found that if you see other airplanes that appear to be moving, you will not collide.
If you see another airplane and it does not appear to be moving, you are on a collision course.
What I have concluded from that is, you will not detect the one that is going to hit. That is why this one got so close before they saw it. It had to appear to be moving and so would miss.
Or you're following the other airplane.