My theory:
I think that the hoteliers are involved. Either on their own or with others. They are the only ones who knew that Tony and Ollie we're going swimming and had access to the yellow scarf afterwards. Silvie was also the one who tempted Tony back this time (I like the theory that she wants the truth out now Alain is dead, she maybe planted the evidence trail- scarf, drawing in basement, video).
If Alain and the Mayor are brothers, the mayor talks about a niece he rarely sees now, which would be Alain's daughter. Maybe she was childless and Ollie was taken for her. She might not be Silvie's daughter, hence why she wants the truth out. Maybe she is Monique, who wanted to see Ollie's father so sought him out.
Maybe Alain was part of the peadophile ring (mayor, Ian, Vincent, Romanian cleaning man) and used these contacts to help him after he had taken Ollie (or Silvie had (could she have taken the fox from their room and used that? Emily had problems finding it again, so miht have been moved?)). Maybe Alain kept them quite about Ollie's whereabouts under the threat of exposure.
The lady in the cafe with the boy in episode one wasn't Monique was it??
----
Random other thoughts- Tony and Ollie were both carrying a bag when they went into the bar. Ollie's bag had been dropped outside the bar and was picked up by Tony and droppe again in the toilets. But Tony wasn't carrying his bag anymore then or afterwards? What happened to his bag??!
I think the broken tv might be significant as someone else said.
They couldn't call it The Disapeared as that was the name of the French drama about the children who drowned and then came back.
I don't want spoilers!
I want the programme to come to a satisfying and final conclusion. No more series.