The following page of the Final Fantasy 7 Remake guide provides a description of Discovery - Johnny's Stolen Wallet, available in Chapter 14 In Search of Hope.
While visiting the city in Chapter 14, you will meet Johnny. He will give you the Tomboy Bandit mission. During it, you will complete Discovery Johnny's Stolen Wallet - you can't miss it.
During the quest, go to the Colosseum and defeat Hellhound and Beastmaster.
After defeating the opponents, return to the church. Kyrie and her grandmother are waiting for you. This part of the mission will begin a Discovery. To complete it, you have to go to Johnny and talk to him.
Reward: You get The Johnny Experience trophy for watching all the cut-scenes with Johnny.