His theory centres on a bedroom door which he remembers finding ajar in the Praia da Luz flat, a family friend said.
The friend added: "Initially he thought that Madeleine might have got up and gone to the toilet or gone to get a drink or something, but now he thinks that the abductor must have been in there hiding. He believes he came in, opened the door and didn't have time to close it. Once Gerry had left he would have known he had to come out very quickly, and because Gerry went out through the patio door, went out through the window.He has no proof but that is what he believes."