The Chink in the Armour by Marie Belloc Lowndes

But the longest night, the most haunted night, and Chester's night had indeed been haunted, comes to an end at last. After he had had another bath and a good breakfast he felt a very different man to what he had done three or four hours ago, lying awake in the sinister, companioned atmosphere of his bed-room at the Pension Malfait.

Telling his courteous landlord that he would not be in to luncheon, Chester left the house, and as it was still far too early to seek out Sylvia, he struck out, with the aid of the little pocket-map of the environs of Paris with which he had been careful to provide himself, towards the open country.