Why would dropping a passport on a public highway be so suspicious?
Anyone in Chalons could have gone along that road. And the presence of an item does not really give enough info about when it was dropped, so I don't think that just something linking an ID to an unspecified time on a road is incriminating enough.
But something about it made Z bag it immediately, and pass it to Suri. What would he need to see, not just be told about?
Sieg was watching the road, and I think the natural assumption is that he was keeping an eye on the police operation. This might or might not include waiting until they had moved away to retrieve an item known to be missing.
Question: did that road junction have a road block in the aftermath of the abduction, or be a control point for people entering the town in the immediate aftermath (remember Emily's parent's arrival). Did someone put something in the drain deliberately to hide it when stopped at that junction?
Or was Ollie transferred between cars there, and something dropped by someone during the transfer? Less plausible I think, as that would mean someone missed an item and could narrow down its location accurately enough to send Sieg to find it and clean up.
I was wondering if Malik dropped something, and Ziane recognised it as his (hence saying that he was now free of obligation, because at that stage he believe they had reached an M.A.D. state). But as Sieg works for Caid, I think that idea would fail.