Aaron writes...

Well, I guess I'll take a shot at this, before more eloquent guys like Aris and Airwalker take it.

The Hyena Contest.

I think the twins had a very... strange childhood, which didn't impress them with the fundamental goodness of humanity. At the end of the day, each is probably the only one that the other even partially trusts.

They're both terrified of being alone, which is why a psychopath and a sociopath stay together. Hyena is smiling because she knows, subconsciously, that getting captured is the only way to stay with her brother.

(Or maybe Hyena was so glad not to be blind after seeing a million candle-power searchlight through her NVGs, she was happy to see anything, even a cop)