Secret Sam Fisher outfit:
The starting location for the Government Plane Wreck (fifth) expedition is randomized. Look for a rusted (brown) plane wreck. It is the body of the plane and still has two wings on it. There is also a blue ladder leading up inside the wreck. Enter the plane wreck, and interact with the laptop on the ground level (next to the only enemy on the ground level) to unlock a secret vault upstairs. Go to the upper level of the plane wreck, and interact with the vault door at the end of the room to find the secret Sam Fisher outfit behind the vault door and get the "Paladin's Secret" achievement.
Accomplish the indicated achievement to get the corresponding number of Gamerscore points:
Honorary Scout (10 points): Gather intel 10 times by talking to Wiki-Bean-ia scouts. Host only.
Yoink (10 points): Loot a Highwaymen Supply Drop. Be careful, it won't be unguarded!
Forager (10 points): Gather 15 plants growing in Hope County. It's the closest thing to a pharmacy we have nowadays.
Reduce, Reuse, Recycle (10 points): Scavenge an Outpost. You can always reclaim it. Host only.
Home is Where the Part Is (15 points): Upgrade Prosperity's infrastructure for the first time by investing in the right parts. Host only.
Sidecar Sidekick (10 points): Drive with Timber for a total of 5 minutes. Just don't take him to the vet.
White Gold (15 points): Commandeer an Ethanol tanker and bring it back to one of your outposts.
Finders Keepers (15 points): Complete 3 Treasure Hunt missions. Host only.
Safekeeping (10 points): Picklock 5 safes. With no more banks, it's the closest thing you'll get to experiencing a heist.
Closed for Applications (20 points): Recruit all Guns For Hire. Host only.
How's it Look, Doc? (10 points): Craft 10 Medkits. They'll definitely come in handy.
Kill or Be Killed (20 points): Kill a Monstrous animal. It'd do the same to you given half the chance.
Before (20 points): Find all the Dear Photograph locations and see Hope County as it used to be. Host only.
Audiophile (20 points): Rediscover some sweet tunes for the new world. Find all hidden MP3 players. Host only.
My Little Fortress (15 points): Make Prosperity better than ever! Build all Infrastructure upgrades available. Host only.
Master Skinner (15 points): Collect at least one skin from every type of animal in Hope County. Not creepy at all.
Hit the Road (50 points): Complete 7 unique Expeditions on the Rank III difficulty.
Pure Ninja (30 points): Liberate a Rank III Outpost while staying undetected.
All Your Bases (30 points): Liberate all Outposts at least once on Rank III.
Weaponsmith (10 points): Craft 3 weapons. They don't need to look good, they just need to get the job done!
You're a Catch (10 points): Successfully catch one of every type of fish in Hope County. Get those omega-3s.
Save Your Bacon (10 points): Recruit Horatio. Don't mind his temper, he can be pig-headed sometimes. Host only.
Poof! (10 points): Use 3 smoke grenades.
It's Super Effective! (15 points): Headshot an Enforcer with Piercing Ammo.
Have Buddy, Will Travel (10 points): Complete 3 Expeditions with a co-op partner.
Bring a Knife to a Gun Fight (10 points): Kill 25 enemies using any type of takedown. Put that railroad spike to good use!
Captain's Courageous (20 points): Kill your first Enforcer. I'm sure this will send the Highwaymen a good message.
Good Job, Cap (10 points): Kill 5 enemies by throwing a shield at them.
Stack 'em Up (10 points): Purchase the 5th tier of a stackable perk. You can never have enough.
Fly, You Fools! (15 points): No wings means less drag, right? Take a short flight in the wingless plane.
Buzzkill (10 points): Kill 10 brawlers with any saw launcher. They never saw it coming.
Legend Has It (20 points): Craft your first Legendary Weapon or Vehicle and start a legend of your own.
Tip o' the Hat (15 points): Knock an enemy's helmet off with a charged melee attack.
Perk-olate (10 points): Unlock the first level for all Perks.
Archery Expert (20 points): Kill an enemy more than 100m away with a basic arrow.
Get to the Point (40 points): Finish off an Enforcer with a bayonet.
Additionally, there are 14 secret achievements:
The Meaning of Prosperity (20 points): Help Carmina get back to her mother in Prosperity. Host only.
Prospering (50 points): Upgrade and secure Prosperity. Host only.
Return to Eden (50 points): Bring the Father back to New Eden. Host only.
Problem Solver (50 points): Solve the biggest problem in Hope County by defeating the Twins. Host only.
The End of Eden (100 points): Decide the fate of the Father. Host only.
A Graceful Return (20 points): Recover Grace's prototype gun and welcome her back to Prosperity for good. Host only.
Coming Up With the Goods (20 points): Return Selene's valuable stash. Host only.
Wiki-Bean-ia in Business (20 points): Help Bean get back his business plan so he can set up his intel network in Prosperity. Host only.
The Ryeunion (20 points): Reunite Nick Rye with Carmina and Kim. Host only.
Expeditious Retreat (10 points): Complete your first Expedition to force the Highwaymen out of a stronghold.
Paladin’s Secret (10 points): Find Sam Fisher’s outfit in the Government Plane Wreck expedition
Anger Management (15 points): Eliminate 10 enemies within a single activation of Wrath.
Hey Father, Watch This (15 points): Perform an Aerial Takedown after using Leap of Faith.
Springboard (20 points): Survive a 30m fall using only Leap of Faith to nullify the impact.