A B727 APCHING ATL; GA; HAS A TCASII RA. FLC CANNOT FIND TARGET VISUALLY AND APCH CTL STATES THEY HAD NO SUCH TFC.

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WE HAD BEEN RADAR VECTORED TO INTERCEPT THE LOC TO RWY 9R AT ATL. SIMULTANEOUS APCHS WERE BEING CONDUCTED TO RWY 8L; A PARALLEL RWY TO THE N. THE WX WAS ESSENTIALLY CLR AS WE COULD SEE THE ARPT FROM 20 MI AWAY AND HAD THE RWY IN SIGHT. WE HAD INTERCEPTED THE LOC AND WERE TRACKING IT INBOUND; ABOUT TO INTERCEPT THE GS. AS PART OF MY ROUTINE SCAN; I GLANCED AT THE TCASII DISPLAY TO KEEP TRACK OF THE ACFT AROUND US. SUDDENLY A TARGET APPEARED ON OUR SCOPE TO OUR IMMEDIATE L AS A WHITE DIAMOND. THIS WAS SURPRISING AS WE HADN'T SEEN OR BEEN ADVISED OF ANY OTHER TFC IN OUR IMMEDIATE VICINITY; PRIOR TO THIS. I IMMEDIATELY SEARCHED THE SKY WHERE TCASII INDICATED THE TFC TO BE; BUT SAW NOTHING. RECHKING THE TCASII DISPLAY TO UPDATE MY REF I SAW THE TARGET HAD CHANGED TO A FILLED YELLOW CIRCLE NOW; INDICATING A POTENTIAL CONFLICT. ONCE AGAIN I SEARCHED WHERE THE TARGET WAS INDICATED WITH NO SUCCESS. AT NEITHER TIME WAS ANY ALT INFO GIVEN BY TCASII. SECONDS LATER TCASII GAVE AN AURAL 'CLB; CLB' RA; THE IVSI CHANGED TO A CLB COMMAND AND THE TCASII TARGET CHANGED TO A RED SQUARE (IMMEDIATE THREAT). THE FO (WHO WAS FLYING AT THE TIME) IMMEDIATELY PULLED UP AND ADDED PWR. I NOTICED TCASII NOW DISPLAYED A VERT DISTANCE SEPARATION OF 100 FT. SHORTLY THEREAFTER THE IVSI RETURNED TO NORMAL; THE TCASII TARGET DISAPPEARED AND I ANNOUNCED THAT WE HAD CLBED DUE TO AN RA (TO APCH CTL). AN UNEVENTFUL APCH AND LNDG ENSUED. SUBSEQUENT CONVERSATIONS WITH THE SUPVR AT APCH CTL INFORMED ME THAT NO OTHER ACFT WERE IN OUR IMMEDIATE AREA AT THE TIME ALTHOUGH ANOTHER ACFT FOLLOWING US ALSO GOT A TCASII RA. I WAS ALSO SUBSEQUENTLY INFORMED THAT THE NEXT DAY YET ANOTHER ACFT APCHING THE SAME ARPT ALSO GOT A PHANTOM RA.