Possibly ... and if this string IS aviation-related, and if M5 means Marine 5 (transport for 5th in the line of Presidential succession), that would indicate Steven Mnuchin, Secretary of the Treasury. But B-8028 is a Chinese registered aircraft ... I'm having difficulty connecting it to Mnuchin and AF1 (apologies for being a newbie at this).
No, no! thank you! I was raised in general aviation, but not the comms. I think it IS a... flight plan/deadman combo. And AF1 does not HAVE to payload CINC to use the callsign, if he’s secure on ground, as far as I know.