I spent over 2000 hours to discover this hidden scene in Skyrim's main storyline. While searching for information about dragons at the Thalmor Embassy, you will encounter a Nord who is a Thalmor informant talking to one of the Thalmor members. It's very easy to get into a fight at this moment, during which the informant usually dies. However, if you sneak into the dungeon quietly, you can see him later at the Ragged Flagon.