This is another of the Madeleine McCann conspiracy theories that is explored in the Netflix documentary, The Disappearance of Madeleine McCann. Police looked into the case of a man, who was wanted in connection with the sexual assaults of five young girls at Portuguese holiday resorts.
He was described as a 'lone intruder' who attacked the girls in their beds, two of these attacks happened in Praia Da Luz - where Madeleine went missing from. Anthony Summers says in the Netflix documentary: "One startling element is the sheer number of sexual predators in the area at the time."
In this case, the predator had been making his way into houses where British tourists were staying and allegedly laying on beds with children. He was wearing a medical mask, and described as speaking English, but sounding foreign.