While on Taris, Genda was invited to a party in Taris's North Apartments with other Sith Troopers, including Sarna. At the party, Genda eventually got drunk on Tarisian ale and passed out on the floor, a fate shared by all of his companions who were off-duty the next day.
Genda may have met the former Darth Revan and Carth Onasi at the Sith party, although he wouldn't have realized who they were. If he did, then Revan and Carth would have taken the opportunity to steal one of the soldiers' Sith uniforms while all the partygoers were unconscious.
One of the ways to acquire a Sith uniform on Taris is to be invited to a Sith party. If the player is male, Sarna will invite him; if the player is female, Yun will.