A C172 INSTRUCTIONAL FLT TURNS BACK TO LAND WHEN AN ACCESS PANEL IS OBSERVED BELOW THE R WING AFTER TKOF FROM HNL; HI.

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SHORTLY AFTER TKOF; DURING THE CLBOUT AT 800 FT MSL; MY INSTRUCTOR REQUESTED TO 'COME BACK FOR LNDG.' I WAS UNDER THE HOOD SO I DIDN'T REALLY KNOW WHAT WAS GOING ON. I TOOK THE HOOD OFF AND SAW THE ACCESS PANEL (ABOUT 6 INCHES IN DIAMETER) UNDER THE R WING FLAPPING. IT WAS PROBABLY HANGING ON BY 1 SCREW. DEP CTL ASKED THE PROB AND HOW MANY PEOPLE ON BOARD. MY FLT INSTRUCTOR STATED THAT A 'PANEL ON OUR R WING WAS FLAPPING AND WE HAD 2 SOULS ON BOARD.' DEP CTL ASKED IF IT WAS AN EMER. MY FLT INSTRUCTOR SAID NEGATIVE. DEP CTL SAID THAT THEY'LL HAVE A CREW STANDING BY AND CONTACT TWR. WE WERE GIVEN PRIORITY AND CLRED TO LAND. ON SHORT FINAL ABOUT 500 FT MSL THE PANEL THAT WAS FLAPPING FLEW OFF. WE LANDED SAFELY AND WERE GREETED BY CFR WHO THEN FOLLOWED US TO OUR PARKING. NEITHER DEP NOR THE TWR ASKED US TO FILE A RPT. THINKING BACK AT OUR FLT; THE WHOLE INCIDENT COULD HAVE BEEN PREVENTED WITH A BETTER PREFLT. I THOUGHT WE DID A GOOD PREFLT. I MEAN; WE DID EVERYTHING ON THE CHKLIST. NOTHING LOOKED OUT OF THE ORDINARY. IT JUST GOES TO SHOW THAT EVERY SINGLE SCREW SHOULD BE CHKED VERY WELL. I NEVER REALLY PAID MUCH ATTN TO THE PANEL UNDER THE WING. ACTUALLY; I DIDN'T EVEN KNOW THERE WAS ONE UNTIL IT CAME OFF. IT WAS A GOOD LESSON TO KNOW ALL THE PARTS/ACCESSORY PANELS OF THE AIRPLANE TO ENSURE THAT IT IS SAFE FOR FLT.