A DC9-30 WAS DISPATCHED IN NON COMPLIANCE WITH #2 ENG OIL TANK CAP NOT SECURED.

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MET; PARKED AND SECURED ACFT AT THE GATE UPON ITS ARR. CAPT INDICATED HE HAD FLT LOG ITEMS WHICH NEEDED ATTN. ONE ITEM WAS AN OUTBOUND LNDG LIGHT WAS INOP; AND THE OTHER WAS THE #1 ENG NEEDED OIL SVC. BECAUSE THE ACFT HAD ARRIVED LATE AND WAS A QUICK TURN AROUND; THE LNDG LIGHT WAS PUT ON MEL. I SVCED THE #1 ENG AS NEEDED (3 QUARTS). AS IS MY USUAL ROUTINE WHEN CHKING ENG OIL; I PROCEEDED TO ALSO CHK THE QUANTITY OF OIL IN #2 ENG (1 QUART). NOTE: I ALWAYS CHK THE OIL IN BOTH ENGS. WHILE SVCING THE #2 ENG; I WAS CALLED AWAY BY AN ACR CONTRACT FUELER ON THE ACFT TO ADDRESS A PROB WITH OPENING THE FUEL PANEL DOOR. WHEN THIS PROB WAS SOLVED I APPARENTLY WENT BACK TO #2 ENG AND TOOK MY OIL CART AND STAND AWAY. I HAVE NO RECOLLECTION OF REINSTALLING THE OIL TANK CAP OR CLOSING THE COWLING DOOR. THE ACFT THEN PROCEEDED FROM ZZZ TO XXX. LATER WE RECEIVED FEEDBACK THAT THE ENG HAD EXPERIENCED A LOSS OF 4 QUARTS OF OIL AFTER LNDG AT XXX. THE FLT DID NOT INCUR ANY DEVS NOR DAMAGE FROM ITS NORMAL RTE FROM ZZZ TO XXX; AND WAS IN OP THE FOLLOWING DAY ON THE SAME RTE.