he comes back after being in witness protection the whole time. the dr in on it was Dr Greene- and he was told to tell jack that he had peters liver- when it was actually zoes (i think it was zoes).
but do u remember, a few eps ago they were tellin peter that this guy was gettin out of prison or something (gifford), well peter put his family in jail and his wife (giffords wife) was killed in prison. and he blamed peter for her death and thats why he was targetting drew (peters son) and took pics and sent them to witness protection (as it got out somehow that peter was still alive). but the scene that they show in tmws UK ep, with peters dead body was because the witness prot people drugged him to make it seem he was in a coma(i.e. brain dead). then they moved him and a few days later peter woke up and thats when he was told that everyone thought he was dead.