A BEECH 1900D CREW RPTED THAT BTWN TKOF AND 10000 FT A PAX POINTED OUT THAT THE L ENG NACELLE L SIDE WING LEADING EDGE ICE LIGHT PANEL HAD DEPARTED THE ACFT.

Narrative

I WAS ACTING AS FO ON A REVENUE PAX FLT FROM ZZZ TO ZZZ1. I PERFORMED A NORMAL PREFLT INSPECTION ON THE ACFT AND FOUND NOTHING UNUSUAL. DURING THE COURSE OF THE FLT; A PAX APCHED THE FLT DECK TO INFORM THE CAPT AND MYSELF THAT HE HAD SEEN 'A PIECE OF METAL ON THE L ENG FLAPPING UP AND DOWN; THEN IT FELL OFF.' THE CAPT VISUALLY CHKED THE L ENG NACELLE AND COULD NOT SEE ANYTHING ABNORMAL. ALSO; ALL ENG GAUGE INDICATIONS WERE NORMAL. THINKING BACK TO THAT PORTION OF MY PREFLT INSPECTION; I DID NOT FIND ANYTHING UNUSUAL ON THE L ENG COWLING; AND VISUAL INSPECTION OF THE ZEUS FASTENERS DID NOT REVEAL ANY LOOSE SCREWS. A POSTFLT INSPECTION REVEALED THAT THE L ENG ICE-LIGHT PANEL (APPROX 15 IN X 25 IN) HAD BECOME SEPARATED FROM THE ACFT DURING THE FLT. THE ONLY THING I CAN THINK OF TO DO DIFFERENTLY IS TO PHYSICALLY PULL ON THAT PANEL DURING EACH PREFLT. THE L ICE LIGHT HAD JUST BEEN REPLACED AND THE MECH MAY NOT HAVE PROPERLY SECURED THE PANEL AFTER THE REPAIR. THE COMBINATION OF THIS MAINT AND MY NOT FINDING ANY ABNORMALITIES DURING PREFLT MOST LIKELY CAUSED THIS INCIDENT. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR STATED IT WAS DETERMINED BY THE COMPANY MAINT INSPECTOR THE LIGHT PANEL FASTENERS WERE NEVER TORQUED PROPERLY. THE RPTR SAID THE WALKAROUND INSPECTION WAS PERFORMED PER THE BOOK AND EVERYTHING LOOKED OK.