To earn the "Someone's out here." trophy in Firewatch (EU), you reach the end of Day 76. The trophy earned before this is Day 2, so there are several days you need to work through without earning any trophies before you get to this one. Luckily they're all fairly short, but if you need help, check in the spoiler tags.
*** Spoiler - click to reveal ***
Day 3:All you need to do here is collect three wooden planks and repair the window of your tower. The first and second planks are very near the tower stairs, a few paces away from each other leaning against the base of the tower. The remaining plank is over at the outhouse behind the tower. Go back up and fix the window, and finish your conversation.
Day 9 and Day 15:Two short scenes, just respond to the radio and complete the conversation.
Day 33:You'll start outside, away from your tower. Just keep heading north, past the two ropes you set up on Day 2, and eventually you'll reach a supply drop. Take the middle package only once you've unlocked the cache. Finish your radio conversation.
Day 64:Another dialogue-heavy scene. Let the conversation play out as you wish.
After Day 64 you will fast-forward to Day 76. You'll be out heading towards the lake from Day 1, so make your way past the old campfire and down to the lake. Pick up the clipboard near the water's edge, then turn north. Go across the log bridge and pick up the red radio.
Once you're back in control, head north a short way to Cache 305. With the cache still ahead of you, veer right (north-east) and a short way along the path you'll find a new rappel point. Get down there and then head forward (mostly north) until you reach a fence. Either try to smash the padlock with a rock or just yank the door a bunch of times until it's confirmed that you can't get in the fence.
You'll have a long old trip now. You should head back to your tower by going through the canyon and the cave (as you did on the return trip for Day 1). Once you're back at the foot of the stairs to your tower, face away from the structure. You need to hug the left-hand edge of the path and move forward until you get to a zig-zag dirt path. Head down that and across the river, where you can open Cache 307. Head up into the treeline beyond the cache to find an opening. In the wider area, turn left and keep trekking ahead. When you see the Camp Araphoe sign, head to the left of it and push on up the hill, looking out for a "Long Drop Down" interaction marker on your right. Drop down and continue through the shelters, eventually you'll come out in a recently cleared area and pick up an axe and a memo. You'll be able to use the axe to cut through some brush to the west and then fell a tree to create a bridge back across the little ravine.
You can now start heading back towards the tower by retracing your steps. After some time but likely well before you're back at the tower, you're interrupted by a radio conversation and when that conversation concludes, the day is over. Trophy earned!