WastingMyYoungYears
Caid de Cite is a Paris based gang, many of whose members are Romanian. Baptiste knew of police operations against them, including running an officer (Antoine?) under cover ka Leon to infiltrate them. Their exact scope (of types of crime, and geographic spread) haven't been described, but it is international and could be either or both of drugs and people trafficking.
At about the time of Ollie's disappearance, the gang suspected they had a mile and fed disinformation to suspected gang member/s. Leo, who had seen some signs of additional agitation in gang members, received this information and passed it on. This led to a raid on a rural warehouse from which the high value 'goods' (presumed Ollie) were to be staged en route to Thailand. Nothing there, except security camera. Leon's cover blown. He called another meeting with Baptiste, and was murdered en route.
Leon's Romanian girlfriend Rini (also an addict) tried to find out from her Caid brother what else Leon might have known, and her throat was slashed (she lived, but only just).
Roll on to 2014: once location of house known, the van of Karl Sieg, a person associated with Caid, was identified there on the morning after the disappearance. Rini helped Baptiste track him down. In return for massive bribe, Sieg admitted he had been called to the house by the Romanians to clean the cellar. He saw blood stains. He did not see Ollie. He said he saw, but did not clean away, the picture on the wall.