My theory of the day is that it was Sieg. He was low level local French fixer for the Caid, but also a member of the local paedophile ring.
The Caid was doing something else shifty in the house, involving Mark and the boy at the window was James.
Sieg was called in to clean up the Caid stuff, and planted the scarf and the drawing (he had a yellow rag in his cleaning box, and there was crud under the stairs, so would Ollie really have reached the back of the alcove to draw on it, even assuming he brought or found something to draw with), and he hoped to implicate those who had been in the house that night.
He was watching the road by the park simply because he wanted to return to scene of crime, because he knew he was missing something incriminating and was just waiting until the police left to go and look, or he had something in the back of the van that he wanted to hide at the scene after the police had left.
He only gave minimal information to the Hughes (and in such a way to incriminate the Romanians) and only after Baptiste told him it would be inadmissible because of the bribe.