They were doing the things grownups do when they think no one is observing. But they were, of course, observed. River peered through the hatch above them. She had heard their emotions from upstairs, and come down to see why they were familiar.
She heard their thoughts clearly now: Kaylee, relieved that she was worthy of someone's—anyone's—attention; Simon, nervous that he was no good at what he was doing. She'd only seen her brother helpless once before, but this time he wasn't dying, and...was he enjoying being helpless? Improbable.
Kaylee realized that Simon didn't know what he was doing—this amused River briefly, that there was something Simon didn't know—and took control, steering a torrent of emotion into orderly waves that once again, reminded River of something. She thought of wandering through the corridors of Serenity, of seeing Zoe and—no, she would not call her attention to that.
They were almost dancing now, but in parallel with the floor, and the waves of feeling stronger and stronger until River had to climb away from them, distance herself physically so as to not be overwhelmed by the interference.
Finally the tide ebbed, and River crept back to look at them. They were curled up together, Simon staring absently at the ceiling.
Probability of discovery: 63%. River pulled back again. As she tiptoed off, the parallel came to her.
This was one of the limbic responses programmed into her, the hormonal reward for a successful kill. She now understood ways she could experience that reward outside of her conditioning. She thought about mentioning this to Simon later, but decided against it. She had no reason to anger him by revealing that he had been observed.