How to open locked chests in Dragon Quest 7 Reimagined
Polygon reports that locked chests in Dragon Quest 7 Reimagined require the thief's key to open.
The key is located in a chest in Pilgrim's Perdition: Hidden Room (Past) in the Alltrades Abbey region, inside the building on the north side of the map near the water well. The chest sits behind the counter beside a silver map fragment, but the counter clerk will stop you from taking them, saying they are someone's down payment for the room they rented.
To collect the key and the map fragment, progress the main quest until you have freed Alltrades Abbey (Past) and defeated Cardinal Sin. After that, return to Pilgrim's Perdition and the clerk will be gone, allowing you to open the chest.
The thief's key is not consumable, so it can be used to open as many locked chests as you find, and interacting with a chest will automatically use the key.