A FALSE TCASII RA SENDS A DC9 FLT INTO A CLB TO AVOID A POTENTIAL TARGET 5 MI SE OF BUNTS INTXN; PA.

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WHILE LEVEL AT 6000 FT 5 MI SE OF BUNTS INTXN AT 250 KTS CLEAN; WE HAD JUST BEEN HANDED OFF TO PHL FINAL APCH WHEN WE GOT A TCASII RA TO CLB FULL SCALE 6000 FPM WITH NO PROXIMATE TFC PRELIMINARIES. THE FO AS PF IMMEDIATELY BEGAN A MAX PERFORMANCE CLB WHILE I WENT HEADS UP TO ACQUIRE THE TFC WHICH HAD POPPED UP AT 12 O'CLOCK POS LESS THAN 1 MI ALT -03. I DID NOT SEE ANY TFC AND BY THE TIME I COULD GET A WORD IN EDGEWISE ON THE FREQ TO TELL ATC ABOUT THE RA; THE RA HAD EVAPORATED WITH NO 'CLR OF CONFLICT' ANNUNCIATION. (INITIALLY; THE TARGET HAD APPEARED TO CLB WITH US; MAINTAINING A -00 ALT DIFFERENTIAL.) THE FO LEVELED OFF AT 7000 FT AND THEN BEGAN A DSCNT BACK DOWN TO 6000 FT AND ATC SAID THERE WAS NO TFC NEAR US WHICH SHOULD HAVE PROVIDED AN RA. I TESTED THE TCASII (TEST OK) AND ASKED THE LEAD FLT ATTENDANT TO DO A PED WALK; SHE RPTED NO OBVIOUS PED USE. AS APCH HANDED US OFF TO LCL; HE TOLD US THERE WAS VFR TFC BELOW US AT 3500 FT AT THE TIME OF THE RA. WHEN WE GOT TO THE GATE; THE LEAD CAME UP TO SAY THAT A FLT ATTENDANT HAD CAUGHT A LADY TRYING TO CALL HER DAUGHTER ON HER CELL PHONE AT THE TIME WE 'PULLED UP.'