When does laurel find out that Sarah is still alive off of the show arrow?

Laurel Lance discovers that Sarah Lance is still alive in Season 3, Episode 1 of "Arrow," titled "The Calm." During this episode, she learns that Sarah survived the events that were previously believed to have killed her, which leads to significant developments in her character arc and the storyline involving the League of Assassins. This revelation sets the stage for further exploration of their relationship and the impact on Laurel's journey as a vigilante.