She saw a rider moving along a shallow watercourse, and immediately she focused her glasses upon him.

She gave an ejaculation of surprise when the powerful lenses annihilated nine tenths of the distance between them.

One would judge from her manner and her tone that, while she had not been surprised to see a rider, that rider's identity was wholly unexpected.
She watched him until, having reached a certain place where a group of cottonwoods shaded the gully, he stopped and dismounted to fuss with his cinches.

Mary V could not be sure whether he was merely killing time, or whether he really needed to tighten the saddle; but when another rider appeared suddenly from the eastward, she did know that the first rider showed no symptoms of surprise.
She did not know the second arrival at the cottonwoods.

She could see that he was Mexican, and that was all.