This mission unlocks after you take over the Harmanse Gas & Repair outpost.
Approach Seto and accept the mission of reclaiming the stolen items. Find the first person that you are supposed to find and talk to them. Now, you will have to go to the Crazy Cock bar and win, in a game of w Poker, all the money of the other three players. After you have won, approach the gambler and give him his money. In return, you will be rewarded with the first item - a necklace.
Leave the town now and go into the marked area. You will see there a drunken guard, who needs to be killed. Right after you eliminate him and run through his pockets for the item, take to the heels if you want to avoid getting into fight with three other enemies, who will appear here.
The last one of the items can be found on one of the enemies shown in the screenshot. After you retrieve all of the souvenirs, return to Seto to complete this mission.