This is a bit of a buggy achievement, so if it doesn't unlock go back and play through some of the easier Intervention Quests until it does unlock. The video below gives a bit of information. You'll be coming back to these Dungeons when you're a bit more ready. Then go back to the Portal and go back to The Girl's Tea Room. You'll come to a treasure chest that has the EXP Boost Mirajewel is in. Enter the Portal that is on the right. From where you start, go left and head towards the screen. This is the EXP Boost Mirajewel. To get this Mirajewel, you're going to go to The Girl's Tearoom and you'll see three Portals. You're going to go and open a single Treasure Chest that will make the level grind quicker. The Most Important Thing You Could Do Right Now If you need to go hunting for treasure chests, equip a Stealth Mirajewel (to limit enemy encounters) and have a Mirage that has the Flee ability (such as the Black Chocochick).
This means that there is an unopened chest in the area you are currently standing in. This means a chest is in the dungeon you're in, but not in the area you are currently in, so proceed through the area, repeating using the tracker.
This means the area is clear of unopened chests.