While this isn’t a quest, it’s lucrative enough that it deserves a mention in the guide. Not only does it score you some prime loot, but you’ll also get to sniff out a Remnant puzzle, which makes it very worth doing.
To start out head to the Forward Station each of the Pelaav Research Station. From this Forward Station turn southwest, leap over a large, stand-alone boulder and continue southwest until you find a Remnant wall. Follow this wall west, then south, and when your progress is interrupted by some rocks turn west again and leap over them, then turn south one more time to find a cave with a lone Roekaar Raider in it. Kill him, then head over to where he was standing to discover a Remnant Console, which invitingly rises to meet you. Interface with it, then scan the ground to find a conduit, a yellow line of goodness which will lead you back northeast to the Remnant wall you followed earlier.
Note: This treasure hunt isn’t available when you first land on Havarl, as the cave mentioned below is unoccupied, and in place of the Remnant Console you need to activate you’ll find merely a stone. Considering where it takes you, it likely becomes available after extending the bridge across Havarl’s trench, or after reactivating the vault.
Follow the conduit until it ends at the wall, or rather, a diagonal support jutting out from the wall, which you may well have passed under a moment ago. Search the western side of this support to find some platforms sticking out from it, which you can leap up to. Leap up two such platforms, turn south and leap up two more to reach a platform with another Remnant Console on it. Interface with this one and note the line of light that comes off it, indicating - as in the Voeld vault - what it activates.
Drop down off this Remnant structure and make your way through the jungle to the west, past the cave you were in a moment ago and to another Remnant support along a wall, just south of a ruined Angaran highway. This time the platforms you need to leap up are on the western side of the support, so leap up them, turn south, leap up two more platforms, then activate a Gravity Well to propel yourself east, northeast acros Havarl. When you land interface with a third Remnant Console to pop up a Remnant puzzle. Solve it and note the line of light leading further east, across Havarl’s trench. Activate the nearby Gravity Well that opened up to ride across to a small chamber which happens to house a container, inside of which you’ll find much good loot, including the Fusion Mod of Adrenaline I .