AN APPARENT LOSS OF SEPARATION SIT BTWN AN ACR MD80 ON AR1 AND MIL C9 SPILLOUT FROM W-158.

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TORRY INTXN ON AR1 CRG 090 DEG RADIAL/137 NM; WE WERE AT FL310 ALONG AR1 WHEN A LARGE WHITE ACFT IDENTED AS A MIL DC9 CAUSED A TCASII TA WITH THAT ACFT DISPLAYED AT OUR ALT. HE CAME FROM OUR 2 O'CLOCK POS HEADING TO 12 O'CLOCK POS. WE SAW HIM AS HE TURNED AWAY. CLOSEST POINT WAS 3- 3.5 MI ON THE TCASII. HE THEN TURNED AWAY FROM US AND RAPIDLY DSNDED AWAY FROM US. ATC WAS NOTIFIED AND MY CAPT SPOKE TO A MIAMI SUPVR. NO EVASIVE ACTION WAS TAKEN. DID BOTH OF OUR TCASII SAVE THE DAY ON THIS ONE? SUPPLEMENTAL INFO FROM ACN 396252: WHILE CONDUCTING A POST MAINT CHK FLT IN W-158; SEALORD TOLD US TO DSND IMMEDIATELY TO FL240. WE WERE CONFUSED AS TO WHY BUT COMPLIED WITH THEIR INSTRUCTIONS. THEY LATER TOLD US WE WERE OUTSIDE THE W- 158 AREA. WE CHKED OUR POS AND SHOWED US IN THE AREA. OUR MISSION CONTINUED FOR APPROX 20 MINS. WE WERE TOLD TO RETURN TO BASE. SEALORD (FACSFAC) SAID WE WERE OUT OF THE AREA. UPON RETURN TO BASE; THE CAPT TALKED TO FACSFAC AND THE FAA. EVIDENTLY THERE WAS ANOTHER ACFT WHO WAS ON AR1 AND HAD A TA GO OFF. THEY COULD HAVE BEEN OFF COURSE. ON OUR RETURN TO BASE; THE XPONDER WAS RPTED AS INOP BY THE FIRST APCH CTLR AND THE FINAL APCH CTLR SAID IT WAS RECEIVING OUR XPONDER LOUD AND CLR.