AN MD80 WITH A CABIN WATER LEAK IS DEFERRED PER THE MEL. TECHNICIAN ACCOMPLISHED ALL SPECIAL PROC ITEMS EXCEPT PLACARDING THE EXTERIOR WATER SVC DOOR.

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I WAS ASSIGNED TO MEET ACFT INBOUND WITH AN UNKNOWN WATER LEAK. ARRIVED AT ACFT TO FIND WATER ON ACFT INTERIOR; FROM FORWARD EDGE OF L-1 DOOR; ON AISLE OF CLOSET; AND IN AISLE BY FORWARD LAVATORY. THERE WERE ALSO 2 OTHER INBOUND ITEMS; MINOR CONDITION AS I RECALL; A SEAT AND READING LIGHT; I BELIEVE. I WAS INFORMED BY THE CREW THAT THE WATER TO THE LAVATORY IS ON PLACARD INBOUND AND THE WATER TO THE FORWARD LAVATORY WAS SHUT OFF. I CONFIRMED THE WATER WAS SHUT OFF TO THE LAVATORY. I SURMISED THAT THE LEAK WAS UPSTREAM OF THE SHUTOFF VALVE. I COMMUNICATED WITH MY LEAD THAT I THOUGHT THE ACFT SHOULD BE REMOVED FROM SVC. HE INFORMED ME THAT IT WOULD BE A TECHNICIAN DECISION. IN A FEW MINS (APPROX 5 MINS) A TECHNICIAN ARRIVED. HE INSPECTED THE SAME AREA I DID; AND DID NOT FIND THE LEAK EITHER. HE (TECHNICIAN) PLACARDED THE COMPLETE WATER SYS; AND DRAINED THE SYS. I CALLED MAINT CTL FOR A PLACARD OF THE POTABLE WATER SYS. MEL WAS ISSUED. I DRAINED THE WATER SYS; COMPLETED THE PLACARD PAGE ENTRY. I DID; HOWEVER; FAIL TO COMPLY WITH THE MAINT PROC MANUAL. IN MY HASTE AND CONCERN TO GET BOTTLED WATER AND SANITARY WIPES TO THE ACFT FOR DEP; I DID NOT APPLY AN INOP 'DO NOT USE' PLACARD ACROSS THE WATER SVC DOOR ON THE ACFT FUSELAGE SKIN. THE ACFT DEPARTED; AND I WAS MADE AWARE OF THIS INCIDENT.