Amidst the Mill's Grist Walkthrough - The Witcher 3

After completing La Cage au Fou, you will watch a flashback of Count de la Croix and Dettlaff's first meeting and their last, where the higher vampire seemed grief-stricken to kill his friend. While there's no objective marker or indication that there's a secret quest at the crime scene, there actually is! However, if you don't complete Amidst the Mill's Grist in The Witcher 3: Blood and Wine straight after La Cage au Fou (and before Where Children Toil, Toys Waste Away), then you will miss it forever, unless you start another playthrough.