This quest can be obtained from Irondale hamlet in the Feldcroft Region at the southern tip of the map. Travel there then speak with Padraic Haggarty at his shop stall. He’ll tell you that he thinks his sister robbed the people of the hamlet.
Wait for night to fall then return to Padraic’s stall. Use Revelio to find some gold on the ground nearby. Follow the trail of gold west to arrive at the culprit, a Niffler’s lair. Rescue the beast using your bag then loot the area making sure to grab the heirloom. Travel back to Padraic and speak with the merchant to tell him the true identity of the culprit. You can then hand over the heirloom to complete the quest. You’ll be rewarded with a Niffler Mask.