My theory is that they are all involved and covering up for each other: Georges, Alain, Silvie, Ian, the garage men, the charity shop woman etc - the whole town. I think whatever it is has something to do with what happened in the past with Charmartain and Ziane (and possibly Baptiste?!). It could also involve Mark and Tony and the FIL in some slightly disconnected way (they provided or supported somehow without being involved directly?). Ollie himself could possibly be connected if the past event with Charmartain happened in 2002- maybe he was illegally adopted from Charmartain or something.
I think the only innocent person is Malik who was a victim of the past and Charmartain, and who watched the massive town cover up. This is why he is a journalist and clinging to the Ollie case. He wants the Ugly truth to come out- not because he wants to make money from his book, but because that was THe Only Way for the truth to come out and for Ollie to get the justice him and his family were denied by this town in he past. I think Malik will actually turn out to be the only good person and the hero.
I like the memory stick in the drain theory. Other possibilities are a diary, an address book or a phone. I agree it has to be something with vital evidence- obvious that it looks vital at a glance, but something that needs to be kept for a while to be studied properly.