Two comments.
A very long time ago, pre TCAS, on the jump seat of a 737, we were given a warning 'opposite direction traffic 12 miles 1000 Ft above you, so we all of us looked out to see if we could see it. For a very short period of time, we did see it, and we were very happy that ATC had confirmed the separation, as it went past us directly above us. The entire event ( it wasn't an incident, so I'm being careful here) was over in less than 30 seconds, given the cruise closing speeds, but it was discussed in some detail for a lot longer than 30 seconds.
Another very long time ago, due to a LGW controller suppressing the Dunsfold transponder returns, a DAN AIR 1-11 positioning from Lasham back to LGW after maintenance was put at the bottom of the TMA, effectively at 2500 Ft. At the time, we were also over Dunsfold at 2400 Ft, just below the base of the TMA. I can assure you, as we were standing my twin Com on a wing tip, we DID see the crew members in the 1-11 as they turned left for the continued approach to LGW. I never did find out if the 1-11 crew saw us, the controller at Lasham was considerably less than happy, as he was giving us a RAS at the time, and due to late detection was only able to give us about 20 seconds warning of 'opposite direction traffic at or close to your level'.
The controller filed an airmiss, and that's how we found out that LGW has supresses the Lasham transponder returns and therefore didn't see us on their scope.
Close encounters do happen.
The event in this thread might not have been as close as we first thought, or it could have been a full blown TCAS event, bear in mind that depending on the closing angle, the flight deck might not have even seen it until they got a TCAS alert, it it was closing from behind, it would have been very visible from the cabin, but not from the sharp end!