Xana was getting impatient, she knew how potent the poison was, she figured how she didn't want anyone to see her yet before she completed her task, she knew Selena would be dead on the ground, she knew the only reason why she had not asked Sarah the second time was because she couldn't possibly figure out how she die from the previous trial. The whole time she stayed in her room, she knew it was a matter of time before anyone would see the tragic death.