Amidst the Mill's Grist

After the mission La Cage au Fou go to Count de la Croix's Mill close from the Geralt's new home at the sign of the boat. There you will meet some bandits. Kill them and go inside the mill. You will find some footprints. Search them and go up teh stairs. There you will find a blood stain. Search it and let Geralt conclude his findings.

Quest can be sometimes bugged so save before going there.