So a certain ‘Margaret Hookham’ (which was Dame Margot Fonteyn’s real name) asserts that “ASIO records show D.D.Thomson was in Adelaide on the night of the 30th November 1948“ (This sounds a little suspect to me, given that ASIO wasn’t actually formed until 1949.) She also claims that Prosper Thomson, D.D.Thomson (who she says was Alf Boxall’s boss) and a man called Thomas Leonard Keane were all at the 115th Australian General Hospital (6th RAAF Hospital) in Heidelberg in 1943.
So I can't find much out about this DDThomson...
But it seems he was at one point a member of the Communist party, he left because he 'disagreed with the leadership' http://trove.nla.gov.au/ndp/del/article/57268449 this is from 1952, so obviously he isn't the Somerton Man.
but are we sure it's "D.D.Thomson" and not "D.F.Thomson"?
Donald (Finlay) Fergusson Thomson, OBE. (26 June 1901 – 12 May 1970)
In 1941 he persuaded the Army to establish a special reconnaissance force of Yolngu men known as the 'Northern Territory Special Reconnaissance Unit', including tribal elder Wonggu and his sons, to help repel Japanese raids on the northern coastline of Australia. In Northern Australia several irregular units were subsequently formed to utilise the local knowledge and bushcraft skills of the local Aboriginal people to provide surveillance of the more remote parts of the coastline, including the 2/1st North Australia Observer Unit based in Katherine, Northern Territory.
In 1943, as the war moved northward from the Australian coast, the unit was disbanded, and Thomson returned to the Air Force. He was badly injured in action in Dutch New Guinea, and spent the rest of the war in hospital before being discharged from the Armed Forces. I don't know which hospital it was that he was in, but as a member of the RAAF it is entirely possible that he was admitted to the RAAF Hospital in Heidelberg, which is near Melbourne.