B767-300 and 300 ER
Flight Crew Air Transport Pilot (ATP); Flight Crew Instrument; Flight Crew Multiengine
Situational Awareness
Deviation - Track / Heading All Types; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Published Material / Policy; Inflight Event / Encounter Wake Vortex Encounter
Flight Crew Became Reoriented; Flight Crew Returned To Clearance
Environment - Non Weather Related; Human Factors
[We] had a deviation of three miles left of course. We had been on a right SLOP (Strategic Lateral Offset Procedure); we had started encountering wake turbulence from an aircraft 2000 ft above. I decided to return to the track to avoid the wake. I deleted the offset and used heading to return to the track and engaged LNAV again. We were on track and LNAV. I had to go to the Lav. When I returned the First Officer (FO) said we were left of track three miles. I saw we were in HDG mode. I immediately turned to the route and re-engaged LNAV. I know we had verified LNAV after deleting the SLOP I think it's possible that I hit the HDG SELECT button as I exited the seat. Should be more aware of FMA (Flight Mode Annunciator) indications when we change seats and positions.
B767-300ER Captain reported a track deviation in oceanic airspace following a wake encounter. Crew thought they were in LNAV but were actually in HDG SELECT.