Pistol Packing

Pistol Packing (or Missing Laser Pistol) is an unmarked quest in Fallout: New Vegas.

Detailed walkthrough
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. 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). Mention the pistol, and the paladin will mention that Initiate Stanton was the last one to check it out, but signed it back in afterwards. It is necessary to do this before talking to Stanton, or else he will claim that he doesn't know anything about it.

Initiate Stanton
Stanton can be found wandering around the first floor of the bunker or at the classroom during daytime. At night, Stanton will be inside the initiate barracks, which is opposite of the long, oval-shaped hallway from the firing range. When speaking to him, mention the pistol he checked out. 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. If mentioning that the pistol has been checked back in, he tells the Courier that he falsified the records. From here, the Courier may volunteer to retrieve the pistol (i.e. the pistol will not spawn in the gulch until the Courier speaks with the paladin at the Range and then Stanton).

Scorpion gulch
Scorpion gulch is just southeast through a broken fencing line from the bunker. After going through the gulch, the missing laser pistol will be on top of a rock, in the middle of the lowest part of the gulch. The gulch will usually have various scorpions roaming around the area. (NOTE: If you enter the narrow canyon, you've gone too far. It blends in well. It's right near the dead wastelander.)

Conclusion
Once the pistol has been obtained, return to Torres. She will have a tri-beam laser rifle and will give it to the Courier as a reward for services rendered. The Courier will also receive positive reputation among the Brotherhood of Steel.

Behind the scenes
The quest title is a reference to the 1943 song composed by Al Dexter, "Pistol Packin' Mama".

Bugs

 * At the end of the missing laser quest, the tri-beam laser rifle received as a reward will be bugged. Do not place it in the same inventory with another tri-beam laser rifle, as it will be replaced by the one received from Torres.
 * The "dead" wastelander could potentially be alive and fight the scorpions that occupy the gulch with an invisible varmint rifle. He will still drop the normal loot upon death.
 * A corpse may not actually be present to mark the rock: if not, it's the large one at the bottom of the "bowl."
 * After speaking with Quartermaster Knight Torres to begin the quest, if asking Initiate Stanton where the laser pistol is before speaking with the paladin at the firing range desk, Stanton denies any knowledge of the pistol, and the paladin at the firing range desk can't be found to correct the quest. Exiting the bunker entirely and re-entering may correct the missing quartermaster's dialogue.
 * The picture of the missing laser pistol on a Pip-Boy will show that of a plasma pistol instead of a laser pistol.
 * The Search and Mark perk does not highlight the missing laser pistol.
 * If deciding to assist Edgar Hardin in his attempt to become the Elder, this quest will automatically be removed. The missing laser pistol will still appear on the rock. The Courier will have to initiate a conversation with Torres, exit that conversation, and start a new one in order to be prompted to return the pistol.
 * The laser pistol may not appear on the rock, therefore voiding the quest. It may be found somewhere else in the gulch (as if a radscorpion moved it).
 * Upon getting the missing laser pistol and returning to Hidden Valley the dialogue options to return the missing laser pistol may not appear. Restarting the game followed by equipping then unequipping the pistol may fix this.