Paladin's Secret is one of the achievements that you can gain by playing Far Cry: New Dawn. You will unblock them during the expedition Wreck of Government Aircraft.
In order to unlock your achievement, you must find Sam Fisher's outfit. It is located in the largest part of the wreckage - a multi-level segment with wings. At its lowest level, you will find a small platform. Probably a bandit will be around there. There is a desk with a laptop there. Approach it and interact. This way, you can open the locker with the costume.
The cache itself is at a higher level. Just go up the stairs next to the platform. Enter them and look to the left. You will see an open capsule with Sam Fisher's outfit.