The Non-Canon Expert explains how Darth Vader first discovered that Luke was using the lightsaber that formerly belonged to the Jedi Anakin Skywalker, a lightsaber he had not seen since it was taken from him by Obi-Wan Kenobi upon Mustafar. This topic is looked at from both a Canon and Legends perspective. The Canon information can be found in the 2015 Canon comic “Star Wars” issue #2. The Legends information is primarily found within the Legends novel “Star Wars: Splinter of the Mind’s Eye” by Alan Dean Foster and the Legends novel “Star Wars: The Rise and Fall of Darth Vader” by Ryder Windham. I hope that you enjoy it!