MOSCOW (Reuters) -Former Russian President Dmitry Medvedev said on Monday that it remained a mystery who was behind a wave of drone disruption in various European countries, but that the incidents served as a useful reminder to Europeans of the dangers of war. Medvedev, now deputy chairman of Russia's Security Council, played down the theory that recent disruption, including to airport traffic in Germany and Denmark, was the result of any actions connected to Russia or forces sympathising with it.