Jack isn't easily given to awe and wonderment -- he was always supposed to just accept things and move on, not dwell on them too much -- and his mind is on the job of finding a staircase. But he keeps remembering the thought that Sinclair was dead, and half-glancing at Sinclair alive and walking behind him, and getting these sudden feelings of relief. It's almost bizarre. There have been plenty of people down here who've made him feel all kinds of things, but relief has not been a frequent offender.

At a broad staircase, he begins to jog up, but then he pauses and looks back to make sure Sinclair is limping up it without issue. He's not letting the guy get away again that easily.