There's a [GPS CACHE] left of the rope attachment pole (near the rock wall), as is a [PLANT]. You can find some [SALVAGE] on the other side. You've also discovered the 'Bunker Vista' day camp here. While standing nearby the rope attachment pole, shoot an arrow in the craggy rock wall up ahead, then climb the pipes to the right, leading into a bunker.
Inside you can spot a poster/flag with a red encircled star. Burn it to get the first of a series: [#1 PREVIOUS INHABITANTS]. There's also a heavy barricade here that you can't open just yet. Collect the [SALVAGE] near the crack in the wall and head through.
Take down the guard at the end of the tunnel and hide behind something to overhear a conversation between several Solarii guards. You can find [ARROWS x2] here as well. Quickly take out the welders to the left and right sides with headshots, then move closer to the other guards. One of them will soon start patrolling the area, and he's armored so you can't kill him with a single headshot. After killing the two welders, immediately move to the platform to the right, where one of them was working. There's another welder down below, and one more on the far left that you can both take out with headshots. Now take out one of your guns and blow the explosive barrel in the center of the area (when one guard has been killed by the armored one, and the other two are standing close). If you can, also open fire on the armored guard to quickly eliminate him. Having killed these eight enemies gives you a small advantage in the shootout.
After killing all of the remaining enemies (one of them has a riot shield), start looting this place. You can find a [GPS CACHE] directly to the right of the crack you came out of (the southeast corner), and there are also some [GRENADES] nearby. On the left side of the area are [SHOTGUN SHELLS] and [RIFLE AMMO]. There's more [SHOTGUN SHELLS] and [RIFLE AMMO] on the right upper walkway where one of the welders was working. To the north are [GRENADES], [ARROWS x4], [SHOTGUN SHELLS] and [RIFLE AMMO x2], as well as another file: [DOCUMENT - #5 WARTIME INTELLIGENCE (SCIENTIST: IN SEARCH OF THE STAR)].
Head through the door to the left of here and approach the stairs, but immediately back away; some Solarii guards will throw an explosive barrel down. After this, dispatch them (perhaps with the help of some grenades), and watch out for one of the guards may approach you from the door as well. You can find [SHOTGUN SHELLS] and [RIFLE AMMO] next to the desk. The room upstairs contains [SALVAGE x2]. As you move around the corner, more enemies spawn in the main hall. Defeat them, then jump over the gap and search the room in the left corner to find [SHOTGUN SHELLS], [RIFLE AMMO], [ARROWS x2] and [SALVAGE].
Swing the pole to make your way to the third floor. Go around the corner and hide behind the concrete pillar. Kill the enemies up ahead (or an explosion might wipe them out). You can find [SALVAGE], [XP BOX x2] and [LARGE SALVAGE] on the other side of this floor. When you reach the roof of the building, search the northern side (near the rock wall) to find a [GPS CACHE]. There are also an [XP BOX] and the [#2 PREVIOUS INHABITANTS] flag on the wall here.
Drop down to find the 'Endurance Overlook' fast travel camp. You can find the [DOCUMENT - #6 ENDURANCE CREW (ALEX: OVER HIS HEAD)] next to the campfire, and there's a [PLANT] next to the truck. At the very end of this building you can find [RELIC - #1 ANCIENT COINS (JAPANESE 2 SEN COIN)]. There's another plant next to the stacked crates outside, as well as a [GPS CACHE] hidden behind them. The [#3 PREVIOUS INHABITANTS] is rather hard to find, so chances are you came here just for this one. It's located all the way up the tower northeast of the camp. Use a burning arrow to shoot it.
Since you can't do anything about those cracks at the rusty double doors just yet, shoot a rope arrow down the platform to the east and slide down. On this platform, search behind the rocks to the left of the pillar, behind the rocks, to find the last [GPS CACHE] of this area. Slide down the rope to reach the Endurance. [100 XP].
This video shows all collectibles in the Cliffside Bunker area. You'll first need the Rope Ascender, so head to the Endurance first. The order of getting the items in this video isn't ideal, so watch it first and collect items later.