Treasure Maps

You'll be rewarded with a Treasure Map when you reach rank 10, and every 5 ranks after that. The map can be collected from either the Post Office or your Camp Lockbox, revealing a chest containing Gold and RDO$. Additionally, Treasure Chests may contain consumable items, ammo or crafting recipes.

As you complete 28 daily challenges you receive a map as reward.
There's a chance you'll uncover Treasure Maps by looting enemies during Free Roam activities.
You can also randomly find maps stuck to trees with a knife. They spawn mainly at night, look for lantern light.
Another possibility is to encounter randomly a Treasure Hunter and buy a map for 5 RDO$ or kill the Treasure Hunbter and loot the map.

To open a Treasure Map, open your satchel. Go to the document section, select a map and view it. A Treasure Map shows you the area where you can find a chest. Within the area there are four possible spawn locations. On only one location you can find a Treasure Chest. Treasure chests will glow orange while Eagle Eye is active.

Note
At Cattail Pond, the chest on top of the railroad tunnel is bugged. The chest is empty at all. Solution: open another map and reopen the Cattail Pond map. The chest will spawn at one of the other three locations. At Burned Town the door of the south-east building can be locked. Switch your session and try again to open the door. To switch sessions in Red Dead Online, pause the game, select Online, and choose Free Roam to load into a new session.

Note

At Cattail Pond, the chest on top of the railroad tunnel is bugged. The chest is empty at all. Solution: open another map and reopen the Cattail Pond map. The chest will spawn at one of the other three locations.
At Burned Town the door of the south-east building can be locked. Switch your session and try again to open the door. To switch sessions in Red Dead Online, pause the game, select Online, and choose Free Roam to load into a new session.