Then, while he could still pick out the red potsherd, he approached the wall, leant over it, and, failing to detect anything with his eyes, passed his fingers down the stones.
They alighted on a nail; a nail thrust lightly into the mortar below the coping stone.

For what purpose?
His blood beginning to move more quickly Claude asked himself the question.

To support a rope?
And so to enable some one to leave the town?
The nail, barely pushed into the mortar, would hardly support the weight of a dozen yards of twine.
Perhaps the nail was there by chance, and Grio had naught to do with it.
He could settle that doubt.

In a few moments he had settled it.

Under cover of the growing darkness, he walked to the place at which he had seen Grio pause for the first time.