A B757-200 RETURNED TO THE GATE FOLLOWING A PAX DISCOVERY DURING TAXI FOR TKOF THAT A PORTION OF THE TRAILING EDGE FLAP WAS MISSING AND THAT THE TOP HALF OF THE FLAP HAD DELAMINATED.

Narrative

WE HAD A NORMAL PUSHBACK AND TAXI OUT -- SELECTED FLAPS 5 DEGS FOR DEP. FLT ATTENDANT THEN CALLED COCKPIT TO RPT A PAX CLAIM THAT PART OF THE 'WING' WAS MISSING. WE STOPPED ACFT -- CAPT DIRECTED ME TO PERFORM A VISUAL INSPECTION AFTER FOLLOWING SECURITY PROC. I CONFIRMED THAT A PORTION OF THE TRAILING EDGE INBOARD FLAP WAS MISSING AND THAT THE TOP HALF OF THE FLAP HAD DELAMINATED AND SEPARATED FROM THE ACFT. WE RETURNED TO THE GATE. BOTH THE GND CREW AND MYSELF HAD PERFORMED AN EXTERIOR CHK OF THE ACFT. WE DID NOT NOTICE THE DAMAGE WITH THE FLAPS RETRACTED AT THE GATE. PAX NOTED SEEING DAMAGE ON PUSHBACK. THE ACFT WAS REMOVED FROM SVC FOR REPAIR. THE MAJORITY OF THE DAMAGE WAS ON THE TOP SURFACE OF THE FLAP ASSEMBLY. IT WAS A CLEAR SUNNY DAY AND THE MISSING PIECE OF THE FLAP WAS RIGHT BY THE FAIRING. IT IS POSSIBLE THAT WITH A COMBINATION OF THE SUN IN OUR EYES AND A SHADOWING EFFECT FROM THE FAIRING COMBINED WITH THE FLAPS BEING IN THE RETRACTED POSITION; THE DAMAGE WAS MISSED ON THE EXTERIOR WALKAROUND.