The Old Library (Side Quest)
This quest becomes available during the 'Worker's Lament' quest in Letopolis.
Read the marked history in the revealed library to learn about the Sword of Ptah.
Happily, the search area for this wonder is located just next to the Fast Travel point in this area, making it easy to get to if you've already been, and easy to find if you haven't.
Wander the area briefly to trigger a cutscene with a rather defensive archaeologist.
He'll give you a tip on a bandit gang to the north which may have taken the blade. This seem dubious, considering how long ago that library must have been buried (and not even considering how long the papyrus was in the library before that). Still, a lead is a lead.
The bandits camp at the Dasos Hideout, near the river to the north.
Head up and scan the area. Ignore the main camp for now- the objective is actually most reachable through a rocky cave opening slightly to the south- marked by a stairs icon and, of course, the objective marker.
Leap down, and roust the hyenas that lair here.
That done, you can snag both the first treasure for the location objectives (red box), and the Sword of Ptah treasure (white box).
Claiming the sword completes the mission, though you may still want to complete the Hideout while you're here.