In the sidequest "Robbing a robber" you help Mithrel in Bavelock to get his stolen bag back from a Bandit.

Talk to Mithrel and he will tell you that a bandit earlier had robbed him of a bag of supplies he was carrying.

Enter the Overworld screen to find him. The first bandit you encounter will be scripted as the one you are looking for. Defeat him and you get Mithrel's bag.

Talk to Mithrel again and receive a reward of 2 Potions.