B757-200
Flight Crew Last 90 Days 200; Flight Crew Total 14860; Flight Crew Type 4360
Inflight Event / Encounter Wake Vortex Encounter; Inflight Event / Encounter Weather / Turbulence
General None Reported / Taken
Environment - Non Weather Related
We were flying along at FL380 in relatively smooth air; with scattered high clouds when we suddenly encountered a moderate bump--only very light bumps before and after. We evaluated the weather (cloud layers); nearby traffic (wake turbulence) and engines (compressor stall(s)) for possible causes. We could see no evident causes; however; since there were no indications of turbulence (contrails--for wake turbulence possibilities) I reported the event to ATC and asked for further ride reports ahead. ATC said there were no reports of turbulence ahead beyond occasional light chop. We elected to leave the seat belt sign off and I asked the Captain if he agreed that I should say something to the passengers (who undoubtedly were wondering what happened). I explained to the passengers that it was an example of 'unexpected turbulence' that I warned them about when I turned off the seat belt sign earlier; that there was always the possibility of unpredictable turbulence from wind; weather; or airplane wakes enroute; but that we had no indications of weather; wake or clear air turbulence ahead so we were leaving the seat belt sign off. When I finished speaking over the PA; and reconfigured my Audio Selector Panel to maintain my listening watch with ATC; the Captain informed me that ATC had just informed him that an A380 had recently flown near our vicinity. I do not know specifics of distance; altitude; or time.
B757 First Officer reported encountering possible wake turbulence at FL380. ATC reported an A380 in the vicinity.