Just thinking about WHY Beth might have been so positive Natalee was kidnapped...first, she was certain something bad had happened to Natalee, because no way would Natalee be late to get with the group for her return flight, because Natalee is so organized and is NEVER late. Second, when Beth started to think of what bad thing might have happened, she cannot bring herself to assume Natalee is dead. So the only thing her mind will accept is that Natalee is being held by someone against her will, but is very much alive. This might be all there is to what Beth is trying to say right now. If there is anything more, she is keeping it to herself for strategic reasons.
I wonder if the chaperone happened to say on the phone that, in checking around for Natalee, they had heard that she was last seen getting in that car with three guys.