Hydrax Roman Camp (Location)
A border fort in Marmarica. One of the possible Centurion locations in the His Secret Service side quest).
As usual with these border forts, the treasure is on the second floor of the building, and the captain likes to watch the front gate himself.
Come in over the south wall, and climb the tower.
Slip into the second-floor room. The camp's treasure is in a red chest in the corner.
As for the captain, you can headshot him from here, but an even sneakier way is to whistle him over to the long grass out front of the fort.
This allows you kill him safely out of sight of his compatriots, for mission completion.