A320 FLT LOST A PIECE OF ENG COWLING DURING INITIAL APCH TO ZZZ.

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OPERATING AS PIC ON ACR FLT XX LAS-ZZZ. DURING INITIAL DSCNT TO ZZZ; FO AND I FELT A SLIGHT VIBRATION OR SHUDDER. SINCE THERE WAS NO INDICATION OF AN ENG PROB; WE ASSUMED IT WAS WAKE TURB FROM THE ACFT AHEAD OF US. THE REST OF THE DSCNT AND LNDG WERE UNEVENTFUL. PARKED AT THE GATE; MY FO DID A POSTFLT INSPECTION OF THE ACFT AND FOUND NO PROBS. IN ADDITION; THE ACFT WAS BEING TURNED OVER TO CONTRACT MAINT WHO ALSO WAS GOING TO DO A THOROUGH POSTFLT INSPECTION; SINCE WE HAD RPTED A POSSIBLE BIRD STRIKE. RPTING FOR DUTY AT XA15 THE NEXT DAY; I WAS INFORMED THAT THE FAA HAD DONE A RAMP CHK AND FOUND THAT A PIECE OF SHEET METAL HAD COME OFF A SECTION OF THE R ENG THAT COVERS HONEYCOMB AROUND THE INSIDE OF THE NACELLE. DURING THE POSTFLT INSPECTION; MY FO DID NOT INDICATE ANYTHING WRONG WITH THE ACFT. HE DOES A VERY THOROUGH PREFLT AND POSTFLT INSPECTION; BUT DID NOT SEE THE PIECE OF METAL THAT CAME OFF BECAUSE OF ITS LOCATION BEHIND THE FAN BLADE. IN THE FUTURE; IF ANYTHING IS SUSPECT; I WILL ALSO DO A POSTFLT INSPECTION JUST TO MAKE SURE NOTHING IS MISSED; REGARDLESS IF MAINT IS TAKING OVER THE ACFT AS WELL.