He remembered how she had refused to climb the peak with Marion, that first afternoon.
You know how self-conscious a secret makes a person.

Jack could think of only one reason why Kate should climb away up there to see him.

She must know who he was, and had come up to settle any doubt in her mind before she did anything.

If she knew who he was, then Marion Rose must have told her.

And if Marion Rose had gone straight and told her friends-- Jack went so far as to pack everything he owned into his suitcase and carry it to the niche in the ledge.