The question I have is how many aircraft in vicinity are not seen in the visual lookout method?
No one here can say.
However, you hear reports from US where they have a much more extensive electronic traffic system used by GA aircraft, that pilots are amazed by how much traffic is not seen in the visual lookout system. Yes I know, those pilots are all idiots who don't have a good lookout, unlike the gods who inhabit Australian cockpits.
I suspect the real reason we are pretty safe from collision is not our lookout, but the big sky principle.