Wow, rude, but it does get across that it probably wasn't meant how it came out and so he decides to just ignore that first comment entirely for now. Koushirou looks pretty put together for the situation, mostly since he's very used to the whole long-term survival thing; the worst part is his shoes, which are definitely somewhat dirty after a bunch of running around, especially since they're less easy to clean than his clothes are.

He watches William's movements, not great a that sort of subtle body language stuff, but he's still getting the impression that William is... Agitated, perhaps, is a good word. Of course, it's probably not unwarranted considering the situation, but it's still a little concerning. The response, though, is one Koushirou can mostly understand and relate to, because--

"I want to know too." And more than that, he intends to know. So whether or not William wants a partner in opening it, Koushirou is going to figure out a way to get it open regardless. Might as well work together, right?