But any way you thought about it, it was remarkable that the slight sound outside--the thudding of a horse's hoofs on soft ground, or the letting down of the bars of the corral--should have wakened Injun.

It probably was not the sound so much as the sense of something unusual, something threatening.

Furthermore, Injun had a different way of figuring things from Whitey.

Also he had been awake longer, so his mind had a better start, not being bewildered by sleep.
"They're up to something," said Whitey.
"Um," grunted Injun.
The two men went into the bunk house and soon came out with another man who was fat.

It undoubtedly was Ham.