He will admit about how he and Apprentice Watkins went out to the scorpion gulch for a 'training exercise.' Once he and Watkins realized the scorpions were more than they could handle, they ran away and he dropped the pistol in the confusion. When speaking to him, mention the pistol he checked out. At night, Stanton will be inside the initiate barracks, which is opposite of the long, oval-shaped hallway from the firing range. Stanton can be found wandering around the first floor of the bunker or in the classroom during the daytime. It is necessary to do this before talking to Stanton, or else he will claim that he doesn't know anything about it. Mention the pistol, and the paladin will mention that Initiate Stanton was the last one to check it out, but signed it back in afterward.
After speaking to her, head next door to the firing range and speak to the paladin sitting at the desk there (if the paladin range master isn't at the firing range desk, try going down to the deck below and stepping into the large bedroom where he should be sleeping and speak to him). Knight Torres, the quartermaster for the Brotherhood of Steel in the Hidden Valley bunker, will mention that she is missing one laser pistol from her inventory, which the Courier can volunteer to help find.