They were in the cabin.

He expected to hear the sounds of breaking-in every moment.
But the fact was that one of them (perhaps Fector, who had stolen papers out of desks in his time) knew how to pick a lock, and apparently was provided with the tools.

Thus while Davidson expected every moment to hear them begin down there, they had the bar off already and two cases actually up in the cabin out of the lazarette.
"In the diffused faint glow of the skylight the Frenchman moved no more than a statue.

Davidson could have shot him with the greatest ease--but he was not homicidally inclined.

Moreover, he wanted to make sure before opening fire that the others had gone to work.