I have various theories!
All start with the idea that Ollie left the bar voluntarily to retrieve his scarf from the changing rooms.
The Sieg theory is that Sieg spotted him there and abducted him, stashing him in the back of the ProNettoyage van. He has access to the scarf, and in the time when Ollie is still alive, he learns of the characteristic big-ears drawing (that's the weak but if the theory). Ollie does not survive the night. Whilst working out what to do with the body, he is called to clean up the Clos house (which always used by Caid for some crime that night, Mark being in cahoots and the boy in the window being James) and plants evidence in the cellar.
Vincent saw Sieg with Ollie, but was scared to talk because of his own offending background, and knowledge of Sieg's local French criminal links. Ian knows or guesses this, and therefore pays for an alibi just to get the Parisian cop away from Vincent.
OR: the Caid are trying to abduct James, (as revenge or extortion over Mark) but get the wrong English boy (Ollie at that time being with his father only, so possibly confusable). The alarm goes up rapidly, and they have to dispose of Ollie.