I don't know how this happens, but it happens quite a lot.
Another fascination with TAS is indeed how much traffic does not get called that is clearly evident on TAS - fortunately I have yet to have traffic not called, visible on TAS, that I would have hit.
Perhaps I have been very unlucky, but as I have said before I have had three near misses. In reality if I had done nothing the first would have undoubtedly missed me (althoung interestingly that one was in CAS and was the subject of Controller error, and an apology which I was happy not to take further), the second was OCAS and again I suspect would have been a miss (I wasnt the flying pilot, but the pilot never saw the aircraft until after we had taken avoiding action), but the third did unnerve me.
The other twin passed directly under me with minimal vertical seperation - I would guess no more than 50 feet. Unsurprisngly it all happened so quickly, it was almost surreal. However I found myself contemplating the reprecussions of even two light twins meeting at 200 knots. I still cant fully visualise how horrifying would be the moment of impact never mind the debri below.
I accept not relevant to this discussion as the instant above was in VMC, but the reuslt in IMC would be equally horrifying - and in both cases you would probably know very little about it!