Before we get to digging too deep here, consider this your spoiler warning. If you want to discover all of the secrets of an upcoming mission in GTAV on your own, then tread carefully.
The above tweet comes from a group of folks who have made a hobby out of diving into the code of Rockstar games. When The Doomsday Heist was launched earlier this week in Grand Theft Auto V, the crew got to mining the data for any scrap of extra content that may not have been discovered in-game yet.
According to their findings, a "treasure hunt" will eventually be unlocked in GTA Online, leading players to, presumably, discover a hidden trunk. It's not uncommon for extra code/content to get rolled into something totally unrelated, so we don't expect this treasure hunt has anything to do with The Doomsday Heist directly. Instead, it's more likely the mission will pop up whenever Rockstar decides to activate it, or as part of a promotional push for Red Dead Redemption 2.
The dataminers were able to come up with quite a few extra details, specifically what you'll find in the chest. Apparently, it will unlock an in-game double-action revolver, which are the six-shooters you typically see in westerns or, say, Red Dead Redemption. It will also unlock something in Red Dead Redemption 2 once it finally releases.
That tweet was followed by another with some extra information which, as noted by Kotaku, ties back in to something of an in-game mystery that's been around since GTAV launched.
When folks first stumbled upon the unmarked grave in GTAV, speculation started stirring that it would either eventually tie into an in-game mission or that it was perhaps an Easter egg referencing a deceased character in the original Red Dead Redemption. The latter doesn't really match up, as the location is way off for where that grave would have been located.
The former, however, seems to be right on the money. Those code spelunkers kept on digging, following clues until they were able to figure out where the chain of missions will eventually lead. It turns out the treasure hunt will draw to a close at the unmarked grave, where players will uncover the chest and the in-game loot.
This makes a lot of sense if you've seen The Good, The Bad and The Ugly, a spaghetti western Red Dead Redemption owes a lot to. The treasure in that film could be found at an unmarked grave, just like the one hanging out on a hill in GTAV.