FLC OF LGT ACR ACFT CRUISING AT HIGH ALT OBSERVE A MIL SPC ACFT TAKE EVASIVE ACTION TO AVOID THEM AFTER CTR RPTED 'UNKNOWN TFC.'

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MERIDA CTR CALLED UNKNOWN TFC AT UNKNOWN ALT. WE SPOTTED HIM AT 2 O'CLOCK POS SLIGHTLY LOWER AND RAPIDLY CONVERGING. THE AIR FORCE SPC AIRBORNE WARNING AND CTL SYS ACFT SPOTTED US ABOUT THE SAME TIME AND MADE AN EVASIVE L TURN TO PASS BEHIND AND JUST BELOW US. I DON'T KNOW WHETHER THE AIRBORNE WARNING AND CTL SYS WAS AWARE OF US MUCH BEFORE HE MADE HIS TURN. WE WERE WELL WITHIN MEXICAN AIRSPACE AND THEY WERE NOT AWARE OF HIS ALT OR ANYTHING OTHER THAN POS. THIS EVENT SHOULD NOT HAVE HAPPENED AND WAS CERTAINLY EXCITING TO ANY PAX WHO WITNESSED IT. I BELIEVE THAT THE AIRBORNE WARNING AND CTL SYS WAS NOT WHERE HE SHOULD HAVE BEEN AND HAD HE NOT HAVE TAKEN A L TURN I WOULD HAVE HAD TO PULL UP AND TURN SHARPLY; PROBABLY TO THE R TO KEEP HIM IN SIGHT. SUPPLEMENTAL INFO FROM ACN 238832: WHILE IN CRUISE; MERIDA CTR RPTED FAST MOVING TFC AT 3 O'CLOCK HDG SE WITH NO SQUAWK. MADE VISUAL CONTACT WITH AN MIL SPC ACFT HDG SE AT A SLIGHTLY LOWER ALT (ESTIMATE HE WAS AT FL300). ACFT DEVIATED VERY LATE BY TURNING L; TO PASS JUST BEHIND AT WHAT WE ESTIMATED TO BE LESS THAN 1 MI. THE LATE DEV BY THE SPC SUGGESTS THAT THEY DID NOT KNOW THAT WE WERE IN THE AREA. MERIDA CTR HAD NO RADIO CONTACT; NOR WERE THEY AWARE THAT THE SPC WAS IN THEIR AIRSPACE. THE SUPVR ON DUTY SAID THAT HE WAS GOING TO FILE A RPT TO HOUSTON CTR; AS THIS WAS NOT THE FIRST TIME THAT ACFT HAD FLOWN THROUGH MERIDA AIRSPACE WITHOUT A FLT PLAN; AND WITHOUT COMMUNICATING WITH ATC. I DO NOT KNOW IF MIL ACFT ARE ALLOWED TO FLY AROUND WITHOUT A FLT PLAN OR WITHOUT COMMUNICATING WITH ATC; BUT IF SO IT SEEMS LIKE AN ACCIDENT WAITING TO HAPPEN.