One of the more challenging activities you can encounter free roaming in Red Dead Online is a Gang Hideout waiting to be cleared. These camps of enemy bandits occasionally pop up in fixed locations and clearing them carries with it the chance to acquire some valuable items or treasure maps.

If you intend to complete challenge awards, you'll inevitably be directed towards these hideouts sooner or later.
Gang hideouts can spawn in 24 different locations. These spawn locations are fixed, but there is no guarantee that the camp will be there when you travel to the spawn point. Of the 24 locations, a few are simply out in the open with no props in the game world that would otherwise indicate their presence (and the fact it's a gang hideout) unless the hideout itself has actually spawned in.
Thanks to a Reddit user who goes by Rock1Jump, we have a map showing each Gang Hideout spawn location. The two locations that the author marked as "makeshift camp" are the ones that aren't obvious unless the hideout has actually spawned in.
Here you can check out another, updated map, courtesy of CoolHandLukeZ who updated a map originally made by FuryMexican.
When you clear gang hideouts, there is a chance that the gang leader will drop a treasure map that will lead to you valuable loot somewhere in the game world. The drop rate for the maps isn't known as of present, but it is definitely very low - some players report that after clearing 20 hideouts, they've encountered none, while others found just one after 12 cleared hideouts. At the same time, a few lucky players report drop rates of two out of two cleared hideouts or one out of four hideouts, but these seem like fortunate outliers - especially the former.
Treasure chests can only be looted by the player who has the treasure map. The physical chest itself is visible to all players in the session and can be opened if they find it, but there will only be any loot inside if you have the map pointing to it, otherwise, it will be empty.
Due to the sporadic drop rate, relying on treasure maps from hideouts isn't a good strategy for securing income. Instead, check out our money making guide for Red Dead Online.