There was nothing to indicate that he was even interested.

But Constance knew that that was the method of his shadowing.

Never for a moment, she knew, did he permit himself to look into the eyes of his quarry, even for the most fleeting glance.
She knew, too, that there must be some psychological reason for his not looking at them, as he otherwise must have done, if only by chance.

It was the method followed by the expert modern trailer.

She knew that if one looks at a person intently while in a public place, for instance, it will not be long before the gaze will be returned.