She knew.

She knew because she was thinking of the same things herself.

Of big, fresh, kind Emily Walderhurst lost in her dreams of exultant happiness which never ceased to be amazed and grateful to prayerfulness; of the broad lands and great, comfortable houses; of all it implied to be the Marquis of Walderhurst or his son; of the long, sickening voyage back to India; of the hopeless muddle of life in an ill-kept bungalow; of wretched native servants, at once servile and stubborn and given to lies and thefts.

More than once she was forced to turn on her face that she might smother her frenzied sobs in her pillow.
It was on such a night--she had awakened from her sleep to notice such stillness in Osborn's adjoining room, that she thought him profoundly asleep--that she arose from her bed to go and sit at her open window.
She had not been seated there many minutes before she became singularly conscious, she did not know how, of some presence near her among the bushes in the garden below.

It had indeed scarcely seemed to be sound or movement which had attracted her attention, and yet it must have been one or both, for she involuntarily turned to a particular spot.
Yes, something, someone, was standing in a corner, hidden by shrubbery.
It was the middle of the night, and people were meeting.