It is a closely guarded secret in aviation that most planes are invisible to the Mk1 eyeball. The closer they are to hitting you the less visible they are, and those on a perfect collision course are not visible at all.
But don't tell anybody; I don't want all the retired RAF navigators in the CAA getting a court order on pprune admin to divulge my IP
In the overhead, on average half the planes will be invisible, because they are somewhere behind you. That's why it is such a stupid way of joining. But I've said too much already ...........