The Dead Tree of Ranchiuna (US)

After the credits end, you are free to walk around most anywhere, but for this achievement just go past the skeletons posed on the floor and turn right, then right again. There will be a table with a lockpick kit on it. Pick it up, and go to the locked door directly behind you. Position the lockpick somewhere on the 180 degree arc, and give it a try. Best to start with a NW position, then NE, then North. Mine opened at north--do not know if it is random.

At the end of the credits continue walking through the tomb and you'll find a locked door and a table with lockpicks. Simply pick the lock and the achievement will unlock.

I show this in my achievement walkthrough starting at 1:22:05 in the video.
Intro 0:00
Walkthrough 1:00
Gate 1 8:20
Gate 2 19:10
Gate 3 47:20
Lockpick Secret Door 1:22:05