How to find the A Friend in the Dark Door code at Murmuring Hollow in Horizon Forbidden West Burning Shores

1. Get into the train area

Firstly, while you're staring at the door that you need the key module for, turn right around and you'll notice some rusting train cars behind you. A lot of what we need for this Murmuring Hollow code is behind those trains. So, head back up to the ground level and look to your right towards the campfire. Just to the right of the camp is a yellow piece of metal that you can use to rappel down to the level below. This will get you into the train car area. 

2. Move the first train car

Next you need to do a bit of rearranging. Look at the left-hand car and prize open the doors to get inside. In here, pull the yellow lever to move the car towards the cliff. Head back outside and along the now exposed edge of the train car, you'll be able to grab an energy cell. Grab it, and then head through the stone pillars back into the area where the door you're trying to unlock is. 

3. Move the second train car

Take your energy cell and place it in the train car directly opposite the closed door. Head around to the left and you'll be able to now open the doors to this second train car. Again, use the yellow lever inside to move the train car out of the way - warning, it gets a little perilous. 

4. Get the door code

Moving that second train car has now opened up an entire new area complete with a second locked door. This one needs a code rather than a key module, so let's go grab it. Head back outside again and call your winged mount. Fly up to the upper levels of the ruin and there's a metal panel with a blue cross on it that you can rip open with your Sunwing or Waterwing. Hop off and drop down inside the hatch and look for the elevator shaft. Climb up one level, and there's a data point that you can scan to find the code. 

The door code is 1315.

5. Get the key module

Head back down and use the door code (1315) to open that door you revealed with the train cars. Inside, you can grab a new Horizon Forbidden West ornament for Stemmur, but you'll also need to go and grab the energy cell from the second train car (it just survived) to open up the third door in this area. Within that small room is the 'Auxiliary Key Module' that you need to open that very first door and find out who's behind it. 

6. Find the next door code

Once you've opened up the door and reunited with Gildun - who some of you may remember from Horizon Zero Dawn's Frozen Wilds DLC - you can head further into the ruin. You'll immediately come up against another locked door that needs a code. Head back up the tunnel you came down in, and instead of turning left to head back up, go straight on and scan the data point you'll find there. It'll hint that the code is their birthday - January 17, or 1/17

The code for this door is 117.

7. Reach the next door

Moving into the big room behind that door is, you guessed it, another locked door that requires a code. The door is up on the level above, but just to your right there's another train car. Head around to the left-hand side of it and push it along with Gildun's help until it won't move anymore. In the small room behind the rail car is a cube that you can use to reach the locked door. 

8. Find the door code

Now that you're facing the door, you can try to climb up the yellow railings to the left but they'll break. Instead, go through to the room down the corridor a little to the left. Inside this room, there's a grate that you can pull down using your pullcaster. Crawl in and follow the tunnel along until you're in the next room, then you can squeeze through the door and behind another train car. You'll see some yellow handholds up along the edge just above you, so grab on and follow them around to the other side. Drop down and there's a data point to scan in the room on the right. It mentions "ONE THOUSAND TWO HUNDRED AND FORTY-SEVEN" baked bean cans are left, which feels like a lot of beans, but the perfect door code. 

The code for this door is 1247.

9. Grab the Key Module

But, even though you now have the door code, you still need the key module for this door. Go back onto the balcony outside the room with the data point and you'll see another grate that you can prize open with your pullcaster. Behind it is a yellow wheel you can turn, which it turns out moves the elevator up and down. Let Gildun get into it, lift him to the upper level, and then reunite with him on the other side to push the rail car together. Then, drop back to the ground floor and drag the metal crate from earlier into the lift. Gildun will thankfully get over to the elevator controls, so you can ride up with the crate to the upper level. Use the crate to climb on top of the train car and then run along the top to climb up onto the platform at the other end. In this small room, you can ignite the Firegleam to reveal the key module at last. Unfortunately, this will cause Burrowers to flood into the room below, but at least you can finally open the door when you've taken them all out using the key module and the code - 1247. Then you just need to pull open the grate and find Gildun's friend beyond.