CAPT LEFT OVERHEAD PANEL OPEN AND CAME OFF ACFT ON TKOF.

Narrative

ARRIVED EWR WITH FREIGHT. UNLOADED ACFT AND PROCEEDED INTO THE BUILDING. OUR FUEL TRUCK WAS JUST PULLING UP TO THE ACFT AS I WAS RETURNING TO ACFT. UPON ARRIVING BACK AT THE ACFT REFUELING WAS UNDERWAY. WE WERE USING PORTABLE STAIRS TO REFUEL OVER WING. OP COMPLETED AND I PROCEEDED INTO THE COCKPIT TO OBTAIN OUR CLRNC AND ATIS. THE CAPT COMPLETED THE PREFLT CHKLIST AND SECURED THE CABIN DOOR. WE OBTAINED TAXI CLRNC AND MOVED TOWARD THE RWY FOR DEP. WE WERE CLRED FOR TKOF. I MADE THE TKOF. AFTER LEVELING OFF AT APPROX 5000 FT; THE CAPT DISCOVERED THAT THE OVERHEAD HATCH BEHIND THE COCKPIT WAS MISSING. WE IMMEDIATELY RETURNED TO EWR AND CAPT ADVISED NY DEP THAT THE HATCH MAY BE ON THE RWY. WE RETURNED TO EWR WITHOUT INCIDENT. AFTER DISCUSSION WITH THE CAPT; HE SAID THAT HE HAD OPENED THE HATCH BUT DID NOT CLOSE IT. IT WAS JUST LEFT IN PLACE BUT OPEN. HE HAD FORGOTTEN THAT HE HAD OPENED IT AND DID NOT SECURE IT. IT WAS MISSED ON HIS PREFLT. WE RECOVERED THE HATCH FROM EWR PORT AUTH WITH ONLY MINOR DAMAGE. THE HATCH WAS REPAIRED BY A MECH; INSTALLED AND TESTED FOR OP. WE DEPARTED EWR LATER; WITHOUT FURTHER INCIDENT. CORRECTIVE ACTION: IF SOMEONE OPENS SOMETHING; MAKE SURE IT IS ALL THE WAY OPEN AND OBVIOUS TO SEE; VISUALLY. IF YOU CLOSE SOMETHING; CLOSE IT COMPLETELY.