I put my theory of the day on the Spoilers thread, and it covered a possibility of what happened in the cellar that could mean Ollie was never there.
I think it's OK to repeat here, because it's all based on what we saw in ep6.
My suspect du jour is Karl Sieg. He was low-level, local French fixer for the Romanians. That does not necessarily mean it was his only criminality. And we tend to assume other criminality in the CdeB area.
Ollie was never in the house. The boy at the window was James, because Mark was meeting the gang (though as far as the police were concerned, he was on a family holiday, not vanishing abroad inexplicably).
Something happened in the house that required a ProNettoyage clean up. Sieg had to do it, and used opportunity to cover his own tracks by planting items to lay a false trail.
We saw a shot of the under stairs alcove, and it was cluttered. Why on earth would Ollie have drawn on the wall behind a pile of stuff? Assuming he had something to draw with handy. Why would Sieg have cleaned behind a pile of stuff?
Sieg had a yellow cloth in his cleaning box.
Did he leave clues, in case he needed to incriminate the Romanians (to divert attention from himself, or to reinsure his well-being with them)
Sieg was watching the road by the pool. Returing to the scene? Worried something might be found? Waiting for police to complete search and leave, so he could dispose of incriminating items he still had in the back of the van near the disappearance scene.
Sieg only said the minimum to the Hughes, and even that only when assured by Baptiste it would be inadmissible because of the bribe.