His name arises when a krogan blackmailer, hoping to extort medical supplies from Dr. Chloe Michel, claims to be acting on his behalf. According to Dr. Michel, she and Banes had previously worked together, though they have not been in contact for many years: as far as she is aware, Banes was contracted by the Alliance military for some research in the Attican Traverse. A squad member suggests that Commander Shepard try asking Captain Anderson about him.
If asked, Anderson is surprised at the question: he asks where Shepard heard Banes' name, only to change his mind and decide that he does not want to know. Anderson informs Shepard that Banes has been dead for some time, though this is not common knowledge. His corpse was found on a derelict ship in the Sparta system, but beyond that, Anderson cannot tell Shepard more. He suggests the Commander speak with Rear Admiral Kahoku, as Kahoku's crew found Banes's body. Shepard can speak to Kahoku regarding Banes, only to discover that the marines who found the body are now missing.
Little else is known about Banes, but given the "classified project" Anderson mentions, and the fate of Kahoku's marines, he may have been involved in a project sponsored by Cerberus.
In order to complete that short chase of leads on Armistan Banes, Citadel: Doctor Michel must be completed before UNC: Missing Marines, as after Missing Marines, Rear Admiral Kahoku will no longer be present on the Citadel and the lead will end with Captain Anderson.
Mass Effect 3
If Dr. Michel is invited aboard the Normandy SR-2 instead of Dr. Chakwas, and if Shepard visits the crew quarters directly after the Sur'Kesh mission is completed and Mordin Solus/Padok Wiks is temporarily invited aboard the Normandy, Dr. Michel and Engineer Adams will be having a conversation, and Dr. Michel will clarify that she used to work with a man named Armistan Banes, and she explains that she thought Banes was with the Alliance but Banes turned out to be a Cerberus agent.