Obtained: Colonel James Hsu at McCarran / Vulpes Inculta at Fort Caesar.
Go speak with Colonel James Hsu in his office inside McCarran's hangar and offer him your services to find the mole plaguing the base (Image 1). Then go speak with Curtis in his office at the other end of the hangar to obtain additional information (Picture 2). The captain will send you to Lieutenant Boyd to help you with your task. Go upstairs in the hangar and enter the interrogation room above the captain's office to chat with Boyd. Simply tell her that you want to fix the problem and she will give you her information (Image 3).
Now head to the back of the shed and exit. Once outside, move towards the communication tower to the northwest (Picture 4). Hack the terminal at the entrance to the tower to find out what time of day the mole is likely to show up (Picture 5). Attention, from this moment, it is imperative not to divulge any more information to Captain Curtis under penalty of seeing the mission end in a somewhat frustrating way (Image 6).
Stand in front of the tower and wait until 1am then enter. If the tower is empty, repeat the operation the next day. Inside, you will overhear a conversation between the Legion and Captain Curtis (Picture 7). Wait until he confesses to having planted a bomb on the monorail to intervene and kill him. You can then retrieve the activation code from his corpse (Image 8) and head towards the access to the monorail located just above Hsu's office (Image 9).
Outside again, enter the monorail (Picture 10) and open the ventilation plate at the far left to find the bomb (Picture 11). Use Curtis' codes to defuse the bomb (Picture 12) then do your a Hsu to complete the quest.
Note: Curtis is the mole you are looking for in this mission, if you confess to him that you found clues in the communication tower, you will be unable to prevent the destruction of the monorail. Likewise, if you don't let Curtis indicate the presence of a bomb on the monorail at his touch of the Legion, you will not be able to prevent the bomb from exploding.
Find the mole but let the monorail explode
Blow up the monorail for the Legion