To unlock the Without a Trace trophy in Assassin's Creed Odyssey you must have purchased Episode One of Legacy of the First Blade (or the Season Pass) and have downloaded the episode.
The Episode unlocks a new ability called Death Veil which you need to purchase. With this ability, any enemies killed stealthily vanish in a gold light once prone, meaning their bodies won't be discovered. Pretty handy!
You will probably unlock this through normal play during the course of the main DLC quest, but if not just go to a lower level area and stalk a few guards. It won't take long to rack up 10 silent kills. If you're really struggling purchase stealthy skills such as Critical Assassination or Shadow of Nyx and make sure to use them.
Once you rack up 10 silent kills after purchasing this ability the trophy will unlock.