Thief: The Dark Project Reference
Proceed through Mission 7: The Flooded District, until you arrive at the Assassin's Hideout location in Central Rudshore. Your overall objective at that building is to meet with the primary target, but your approach matters. Sneak inside to avoid alerting your enemies. Once inside, look for a training area. Two assassins are positioned near one another, and one of them congratulates the other on a job well done. "I did not see you approach," he comments, which is the same dialogue you might recall hearing if you completed the stealth training tutorial in Thief: The Dark Project.