I'm thinking Lucas makes it to look as if they were attacked, and Denise is killed. In tonight's episode there is a mention of a prostitute who looks like Denise- she'll will really be the one he kills. He takes Denise hostage to keep her quiet because she knows his secret, then when the prostitute's body is pulled from the water, Lucas identifies it as Denise and everyone thinks she is dead. How we go from Denise in the car, to dead prostitute in the canal is beyond me though. But that's my theory.