Date: 201712
Local time of day: 1201-1800
Location: ZZZ.ARTCC, US
Flight conditions: VMC
Aircraft: B787 Dreamliner Undifferentiated or Other Model
Operator: Air Carrier
Flight plan: IFR
Mission: Passenger
Flight phase: Cruise
Airspace: Class A ZZZ
ATC / advisory: Center ZZZ
Aircraft component: Turbine Engine
Component problem: Malfunctioning
Reporter organization: Air Carrier
Reporter function: Pilot Not Flying; Captain
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Second reporter function: First Officer; Pilot Flying
Anomaly: Aircraft Equipment Problem Less Severe
Detector: Person Flight Attendant; Person Flight Crew
When detected: In-flight
Result: Air Traffic Control Provided Assistance; Flight Crew Diverted; Flight Crew Returned To Departure Airport
Contributing factors: Aircraft; Environment - Non Weather Related
Primary problem: Ambiguous

Narrative: Thirty minutes after takeoff the lead Flight Attendant called the flight deck and reported that one of the Flight Attendants in the aft cabin saw smoke coming from the right engine. All engine parameters were normal as well as all other aircraft systems. I spoke directly with the observing Flight Attendant over the interphone to get as much detail as possible. I then directed the cabin crew to locate any deadheading pilots and have them confirm the situation. Several minutes later; I got confirmation from one of our deadheading pilots that we did have a large plume of smoke/vapor coming from the engine area under the right wing. The exact area could not be determined based on restricted viewing angles from the cabin. I went back to the aft cabin to verify the severity of the smoke using normal flight deck security procedures.We contacted Dispatch and Maintenance Control to begin developing a plan of action while still heading towards [destination]. Due to the complete unknown cause of the smoke/vapor and the fact that it was not dissipating; we all agreed to return to [departure airport] based on time to divert. Because the engine was not degraded; I elected to...just coordinate a return to [departure airport].During the return leg; the smoke/vapor stopped; this occurred about 45 minutes from the initial report from the Flight Attendants. We were also now at a lower altitude; FL 310 I believe. After a discussion with Dispatch; we agreed to request an inspection from CFR (Crash Fire Rescue) once clear of the runway before taxiing to the gate. The return flight; approach; landing; aircraft inspection and taxi to the gate were all uneventful.When I returned to work a couple of days later; I consulted with the 787-flight standards team and had a conference call with GE engine representatives. During that conference call; the possibility of asymmetric contrails (a contrail visible on one side of the aircraft and not the other due to sun angles and shadows) was mentioned. Based on my inflight observations during the event; supplemented by the deadheading pilot's observation; I would not have made a different decision. This decision was made jointly with my First Officer; Dispatch and Operations all in conference. However; during my follow up discussions I realize the asymmetric contrail phenomenon does provide an explanation and is noteworthy of mention across the pilot group as well as the flight operations division.


Second narrative: The Captain decided that he was not comfortable continuing to [destination]. Based on the reports of smoke; I agreed; but note that I never left my seat and thus never saw what was coming out of the engine. All engine parameters continued to be normal... I completed an uneventful landing at [departure airport].


Synopsis: B787 flight crew reported that they returned to the departure airport after what appeared to be smoke was seen coming from the right engine.