British holiday-maker Paul Gordon told police he spotted the man outside the two-bedroom flat's patio doors while his own two children were sleeping inside.
Kate and Gerry McCann believe their daughter's abductor crept into the apartment through the same patio doors, which they left unlocked so they and their friends could check on their sleeping children during their now infamous dinner at a nearby tapas restaurant.
Mr Gordon, 34, contacted British police about his suspicions in May, as soon as he heard that Madeleine had gone missing. He and his family stayed in the apartment immediately before the McCanns.