There are individuals of sensitive mind, and he was one of them, who know well enough when such a total loss has occurred.

It has been well said that the dead are never really dead to us until they are forgotten, and the same applies to the living.

While they remember us, they are never so very far away, and what is more we, or some of us, are quite aware if they have ceased to remember, for then the door is shut and the doorway built up and our hearts tell us that this has been done.
In Godfrey's case with Isobel, not only did the doorway remained unfilled--the door itself was always ajar.

Although seas divided them and over these no whisper came, yet he felt her thought leaping to him across the world.

Especially did this happen at night when he laid himself down to sleep, perhaps because then his mind was most receptive, and since their hours of going to rest must have been different, he being in India and she in England, she could scarcely have been reflecting on him as he fondly believed, at the moment when she, too, entered into the world called sleep.
Therefore, either it was all imagination or he caught her waking thoughts, or perhaps those that haunted her upon this border land were delayed until his subtler being could interpret them.