The most likely scenario seems to be the suggestion made of a technical problem, whoever was flying turned sharp left, possibly to get back to nearest runway, then without further input aircraft continued until fuel used.
The spanner in the works is the last verbal transmission attributed to the second officer - a seemingly 'normal' response after what seems to have been a traumatic event.
How 'normal' an R/T response this would be from a professional pilot I would question. It would surely be relevant to hear his level of R/T on his other flights, much this was in or out of character could say a lot.