Date: 201504
Local time of day: 1801-2400
Location: ZZZZ.Airport, FO
Flight conditions: VMC
Light: Daylight
Aircraft: B767-300 and 300 ER
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Cruise
ATC / advisory: Center ZZZZ
Reporter organization: Air Carrier
Reporter function: Captain; Pilot Not Flying
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Human factors: Workload; Troubleshooting; Time Pressure; Distraction; Situational Awareness
Second reporter function: Pilot Flying; First Officer
Anomaly: Aircraft Equipment Problem Less Severe; Flight Deck / Cabin / Aircraft Event Smoke / Fire / Fumes / Odor
Detector: Person Flight Attendant
When detected: In-flight; Pre-flight
Result: Flight Crew Diverted; Flight Crew Landed in Emergency Condition; Flight Crew Took Evasive Action; General Maintenance Action
Contributing factors: Aircraft
Primary problem: Aircraft

Narrative: On the ground before departure; we were advised that there was a smell in the Cabin prior to Boarding. We thought it might be fumes from the external power cart; and tried it with the APU Off. (Smelled more musty to me though). It was better after a short period of time. After starting engines; we again checked with the Cabin and they said that no odor was present and all was Normal. After reaching cruise at FL380; the Flight Attendants (FA)s called to inform that they had an alarm in the mid galley. They smelled an odor that they thought might be electrical. The Flight Attendants were very concerned and one was in tears. I believe this alarmed the passengers greatly and we elected to divert as a precaution. This was based on the Smoke Detector in the M2 Lavatory activating and the previous odor issues. Smoke was never present; only an odor that could be viewed as possibly electrical or oil in nature. I never really did smell the odor and relied on the other crew members.We selected the Utility Buses OFF initially as the FAs first advised alarm was coming from the Galley; and left them OFF until landing. We deduced from the alarm that it was a Lavatory; which was confirmed by the relief pilot.   The relief pilot was on break and we advised the FAs to have her check out the Lavatory. She did smell the odor but never saw smoke or a heat source. The First Officer was pilot flying; and I was pilot monitoring. The relief pilot returned to the cockpit and took over company communications; as I returned to ATC communications.  Since we were nearly on top of the divert airport; we elected to expedite the descent and landing rather than running a checklist to try to isolate the possible odor source.      The landing was uneventful on Runway 8 after conducting a Visual Approach. We were under Max Landing Weight and the landing was normal. Since the odor had dissipated and the smoke alarm had silenced; we taxied to the hardstand. Company personnel met the flight and the Fire Department personnel made a check of the Suspect Lavatory. I wasn't as concerned as much about the Lavatory as I was with the odor and the ability of the Smoke Detector to detect something amiss.


Second narrative: [Report narrative contained no additional information.]


Synopsis: A B767-300 flight attendants reported a cabin odor along with an activated mid cabin lavatory smoke alarm. The odor was either electrical or petroleum in nature; but no smoke was seen.