POP UP TFC ALERTS ACRS TCASII RA ON APCH INTO DFW IN PROX OF TCA CLASS B AIRSPACE.

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SIT: VISUAL APCH TO RWY 18R AT DFW. WX: 10 NM VISIBILITY WITH A LITTLE HAZE AND A THIN SCATTERED LAYER AROUND 7000 FT. WE CALLED FIELD IN SIGHT ABOUT 10 NM OUT AND WERE TOLD TO 'TURN R TO 160 DEG AND INTERCEPT FINAL TO 18R.' LEVELING AT 3000 FT AND JUST ABOUT TO INTERCEPT FINAL TO 18R; AN RA POPPED UP ON TCASII ABOUT 2 NM OFF OUR NOSE AND APPROX 500 FT BELOW AND CLBING. NO TA WAS INVOLVED EITHER BY TCASII OR APCH CTL. THE FIRST VERBAL COMMAND WAS 'MONITOR VERT SPD' FOLLOWED SHORTLY BY 'CLB; CLB.' CAPT (PF) INITIATED GAR PWR AND PULLED UP MISSING THE TFC BY 200 FT VERTLY AND LESS THAN 1/2 NM HORIZLY. WE LOOKED BUT NEVER SAW THE TFC. WE COMMUNICATED OUR CLB TO APCH CTL AND WERE TOLD THE TFC HAD 'JUST BEEN IDENTED' AND ASKED IF WE SAW HIM. WE SAID 'NO' AND RESUMED OUR VISUAL APCH TO LNDG AT DFW. THE CAPT CALLED THE APCH CTL SUPVR ON THE PHONE UPON LNDG AND WAS TOLD THERE WAS A SHIFT CHANGE AFTER THIS INCIDENT AND HE DIDN'T KNOW ANYTHING ABOUT IT. HE SAID HE WOULD BRING IT UP WITH THE OFF-GOING SUPVR. MY CONCERN IS: WHY WAS THERE AN ACFT IN CLASS B AIRSPACE THAT APCH DIDN'T KNOW ABOUT UNTIL AFTER IT WAS A THREAT TO US. WHY WOULD AN ACFT BE CLBING INTO AN APCH CORRIDOR INTO A MAJOR ARPT. SUPPLEMENTAL INFO FROM ACN 269737: TARGET SHOWED MINUS 200 FT RIGHT BELOW US ON TCASII SCREEN BEFORE PWR CAME UP AND WE CLBED AWAY. WE STOPPED OUR CLB AT 3700 FT AFTER RA CEASED. CTLR SAID HE WAS SORRY HE HAD NOT CALLED OUT TFC BUT TARGET HAD ONLY JUST TURNED ON HIS XPONDER FOR IDENT. CONCERNS: WE SHOULD HAVE HAD TA WITHOUT ALT; WITHOUT A XPONDER ON TARGET PLANE. NOBODY KNEW ANYTHING ABUT THE INCIDENT 20 MIN LATER WHEN I CALLED APCH CTL. SOUNDS FISHY TO ME.