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Return to Sender - A Long Walk
I'm serious about it being a long walk. You'll need to go all across the Mojave Wasteland for this one.
You can start the quest by going to Camp Forlorn Hope. Sergeant Reyes should be in the command tent 24/7. Talk to her at her radio post and she should mention the recent problems that they're having with the radio reports being intercepted. Agree to take the quest to get the new encrypted radio codes.
With the codes in hand, travel to the six different ranger stations. This part of the quest is simple, once you make it to the actual ranger station you just have to talk to the comm officer and give them the new codes.
Ranger Station Alpha - This one is on the path to the New Vegas and fairly close to the trading post and Boulder City. You should be able to just follow the marker.
Ranger Station Bravo - Bravo is far to the northeast and is fairly close to the Bitter Springs base. There's some mountains in the area, but the path to the ranger station should be fairly straightforward and clear. Watch out for fire geckos though.
Ranger Station Charlie - As covered in the link, it's possible, and in fact quite likely, that Ranger Station Charlie will have been destroyed by this point in the game. Don't worry, they accounted for this. After you check in at the other five stations, the quest will update normally and you'll just report the loss to Sergeant Reyes.
Ranger Station Delta - This should be an easy walk, since you can probably get straight to it from Forlorn Hope.
Ranger Station Echo - This one's to the south and close to Camp Searchlight. Be careful of any Legion soldiers and ghouls on the way there, and try to stick to the west of it. The area's a little rocky and the best shot seems to be coming in from the left side.
Ranger Station Foxtrot - This is probably the hardest to reach, since it's off by itself to the far northwest. It's actually really close to Jacobstown, so if you can swing by on your way up the mountain. I had trouble getting to it from the south. The best way seems to be going up the path until you're about even with it. You can then veer off and basically just "climb" down the slope right into the camp. It worked for me at least and seemed to be the path of least resistance.
Once you've been to each ranger station, report the success back to Sergeant Reyes. Things can't ever go as planned though, so we've got a little more work to do.
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Return to Sender - Handling Chief Hanlon
The first step is to confirm that the reports are false. Just fast travel to each of the five remaining stations and talk to the named ranger in charge to confirm that their reports were all quite normal when they were sent out.
Report back to Reyes to get the final step. The only link remaining in the chain is Chief Hanlon at Camp Golf. Walk up to Camp Golf and go into the resort. Hanlon should only be willing to talk to you during the day, while he's out on the balcony by his radio.
Once you mention the reports to him, he should ask for a meeting that's a bit more private and walk down to his office behind the reception desk. Follow him and talk to him once he's inside. Hanlon's been altering the reports to try and demoralize the NCR troopers and force a pullout. For the most part, I'll state that this is a roleplaying choice. Note that this doesn't appear to have any material changes on the overall battle for the Hoover Dam. The damage has already been done.
There are three basic options:
First, you can agree with his logic and agree to cover for him. That basically ends it.
Second, you can refuse to hide his actions and tell him that you're going to report back to Reyes with the truth. If you do this, he'll lock the door after you leave and announce a final message over the PA system before he kills himself. Note that this is the "best" ending from a material point of view. After he fires the shot, the nearest NPC ranger will open the door and secure the body. His Ranger Sequia will be on the ground next to him though. You can't just pick it up, since it counts as stealing, but you can use "Z" to drag it out to a private area and then steal it. Unless you kill a lot of high level rangers, you won't find this gun often. Report the news back to Sergeant Reyes to complete the quest.
Third, if you have killed Caesar already through your own actions, you can use this to convince him that Caesar's legion is doomed to fail. He'll agree to stop.
The very cheerfully named NCR camp, Camp Forlorn Hope, is home to a few quests to help the NCR in their fight against the Legion in the region. I'll cover how to find the Hydra Thief in Medical Mystery and drive back the Legion in Nelson through Restoring Hope.