Something or somebody had impinged upon his intelligence.
He began to look about him, though always in the wrong direction.

Then, convinced that he was the victim of fancy, he spoke aloud as he had a bad habit of doing when by himself.
"It's very curious," he said, "but I could have sworn that Isobel was here, as near me as when we parted.

I suppose that is what comes of thinking so much about her.

Or do people leave something of themselves behind in places where they have experienced emotion?
If so, churches ought to be very full of ghosts.

I dare say that they are, only then no one could know it except those who had shared the emotion, and therefore they remain intangible.