Thank you for your prompt replies – I was not at all familiar with trigraphs - I’m just trying to clear up some details while (still!) waiting for that journo contact re radar track witness.
The words of that terrorist that the govt does talk to springs to mind:
“We haven’t gone away, y’know.”.
Regarding the trigraph, for it to be of use to the next airfield (or ATC center) “…so that its point of origin could be established immediately in the event of an emergency …”, would this mean that the aircraft’s flight plan would have to be known at that airfield (or ATC center) – or would the numbers refer uniquely to an airfield (ie the point of departure) or a particular waypoint?
And would using the trigraph rather than a more generic callsign indicate that there was a problem or that a particular service was about to be requested – eg say a position fix?
(Remember that the last radio call recorded was well away from the departure airfield and was made to Scot Mil at Prestwick which was not the destination.)