Mr Sethi's wife, Jugnu Mohsin, publisher of Friday Times, the weekly newspaper Mr Sethi edits, has told the BBC she fears her husband may have been killed.
The authorities have denied Mr Sethi is being held by police or any government agency. But Ms Jahangir said they have already admitted he is in the custody of the ISI, Pakistan's feared intelligence service.
In Lahore yesterday, human rights activists and journalists backed a national protest against "the government's increasing intolerance of dissent", starting tomorrow, Pakistan's Freedom of Expression Day. The annual event commemorates the "black day" in 1978 when three Pakistani journalists were flogged for opposing the government's forcible closure of a newspaper.
Government sources have said Mr Sethi, a persistently courageous and iconoclastic journalist, was detained because of his "traitorous" tendencies, and his involvement with RAW, the Indian intelligence service, after a lecture he delivered in Delhi on 30 April.
But Mr Sethi's supporters say he has often made similar remarks in Pakistan, and gave the same lecture to Pakistan's National Defence College, where it was warmly applauded. Mr Sethi has been under no illusions, telling an American television network: "Nawaz Sharif [the Prime Minister] does not like the press ... given half a chance he would scuttle press freedom."