[Boone is perceptive, but not exactly intelligent. He isn't likely to draw anything from her behaviour other than "huh, that was weird".]

...Made a promise to him. Long story.

[At first it had been because he'd promised to help Boone find out who sold Carla- he hadn't really believed him but it was his only lead. After revealing that he didn't need his help anymore thanks to Elle, the only way to stop Vulpes from effectively committing suicide to bring the both of them down was to promise that once they got out of that room, he wouldn't kill him.]