The Palace of the Lady of Grace
The temple in the middle of the Aaru afterlife- you'll pass through several times during the main quest.
The officers you need to kill are on the north and south wings of the temple. Use the marked secret passage point to traverse between the two.
You can use the tall flora both by and inside the temple grounds to make your first approach unseen.
Once you've killed one officer, use the secret passage to approach the other. Keep an eye out for the small box of carbon crystals just by the small-hole-you-need-to-slide-through, it's one of the two location treasures.
Kill the other officer, then take to the heights to approach the final treasure, to the west of the complex, in a garden-of-delights type area.
Patrols are frequent here, so either move quickly or snipe proactively to reach the marked red chest, and complete the location.