Backtrack along the trail, and you'll find that it leads to a building relatively close to the scene. Inspecting them indicates that the man was running from his assailant. There are footprints and a trail of blood leading towards the scene. Geralt will also automatically pick up a key from the corpse. Though shallow, it had severed an artery, and the man presumably bled to death shortly thereafter. Inspecting the body will reveal that a sword wound was inflicted upon the man. Inspect the cadaver to begin the quest (See the image below if you are unable to locate the scent/body).
Follow it, and you'll happen upon a man's corpse. Using Witcher Senses will reveal a scent leading a short distance off the path into a grassy area. You will eventually sense the stench of decomposing, human flesh. Taking the Gate of the Hierarch as a starting point, head into the city and follow the main path as it curves to the right around the Rosemary and Thyme/Chameleon. This quest may be started in Novigrad after Geralt's meeting with Cleaver in a shop near Hierarch Square.
