Yes, I suppose Litvinenko did disappear in one sense of the word, i.e. ceased to exist. However, the sense in which this word is usually used in this context,

Yes, I suppose Litvinenko did disappear in one sense of the word, i.e. ceased to exist. However, the sense in which this word is usually used in this context, is different. It usually denotes a forced disappearance, when a person is secretly abducted or imprisoned by a state or political organization or by a third party with the authorization, support, or acquiescence of a state or political organization, followed by a refusal to acknowledge the person's fate and whereabouts, with the intent of placing the victim outside the protection of the law.[1]. For instance, the following can be validly said about Pinochet's regime and government in Chile: "He shut down parliament, suffocated political life, banned trade unions, and made Chile his sultanate. His government disappeared 3,000 opponents, arrested 30,000 (torturing thousands of them) ... Pinochet's name will forever be linked to the Desaparecidos, the Caravan of Death, and the institutionalized torture that took place in the Villa Grimaldi complex.” — Thor Halvorssen, president of the Human Rights Foundation, National Review I'll take doping scandal over institutionalized torture any time.