ACFT EQUIP PROB. EXTERIOR ACCESS PANEL CAME OFF INFLT AND A SMALL DOOR OPENED. ACFT HAD MAINT PRIOR TO FLT.

Narrative

AFTER STARTING INITIAL DSCNT INTO DAB; WE HEARD A RATHER LOUD THUMP. ALL ACFT SYS WERE NORMAL WE THOUGHT A WHEELCHAIR IN THE BELLY HAD FALLEN OVER. AFTER A NORMAL APCH AND LNDG; I DID A QUICK EXTERIOR WALKAROUND AND NOTED NO PROBS; TIRES AND WINGS. WE HAD A CONSTANT SPD DRIVE PROB FOR MAINT TO FIX SO I PLANNED TO DO A THOROUGH PREFLT AFTER MAINT WAS DONE. WHILE WAITING ON MAINT; GND PERSONNEL CAME UP TO THE COCKPIT AND SAID THAT WE HAD A EXTERIOR PANEL MISSING UNDER THE ACFT AND A SMALL DOOR WAS OPEN. UPON INSPECTION; WE FOUND A 2 FT BY 3 FT AIR CONDITIONING SVC DOOR MISSING IN FRONT OF THE WHEELWELL. ALSO A SMALL RADIO ANTENNA ACCESS DOOR WAS OPEN. THIS APPEARED TO BE THE THUMP WE HEARD IN FLT (1 1/2 HRS AFTER TKOF) SINCE BAGGAGE HANDLERS SAID THE WHEELCHAIR WAS STOWED PROPERLY WHEN THEY TOOK IT OUT. IT WAS DETERMINED MAINT HAD BEEN DONE ON THE ACFT OVERNIGHT AND A PLACARD FOR A PACK DOOR INOP HAD BEEN REMOVED. ON MY PREFLT; ALL DOORS AND PANELS WERE UP AND APPEARED LOCKED; BUT IT SEEMS MAINT MUST NOT HAVE SECURED THE DOORS PROPERLY. I KNOW I DID A GOOD PREFLT BECAUSE WE HAD A DEDICATED MECH TO LAUNCH THE ACFT AND I FOUND A LOOSE LIGHT BULB IN THE R NAV LIGHTS THAT HE REMOVED. THE PANEL BLEW OFF WHEN WE CHANGED PITCH AND PWR FOR DSCNT. MAINT PROBABLY DID NOT INSPECT THE PANEL GOOD ENOUGH WHEN THE WORK WAS DONE AT NIGHT. I WAS ON RESERVE AND RECEIVED A XX05 ALL FOR A XC20 SHOW; XC20 PUSH. 2 FORWARD LATCHES WERE UNDONE ON THE OPPOSITE DOOR. IT WAS POSSIBLE I COULD HAVE MISSED THEM. THE OTHER DOOR HAD SCREW FASTENERS AND COULD BE CLOSED WITHOUT KNOWING THE DOOR WAS SECURED.