"When you get up beyond the main strip of the Algarve there's countless isolated properties where Madeleine could be being held."
Edgar, who runs Alpha Investigations Group, was hired by the McCanns to look into Madeleine's disappearance.
The Metropolitan Police took over the case in 2011.
Edgar told the Sun last week the £ 12million Home Office funding given to Scotland Yard to pay for the inquiry was "not nearly enough for a crime of this complexity and scale". He added: "It sounds a lot but it's not."
Scotland Yard's Operation Grange team have been given £85,000 to continue their inquiries until the autumn.
Edgar says there is no evidence to arrest paedophile Raymond Hewlett or Euclides Monteiro, two suspects in the inquiry.
Hewlett died in 2007, aged 64, while Monteiro, 40, died in 2009.
Edgar once said he believed Madeleine was being held in an underground cell or dungeon, like the victims of Josef Fritzl.