You can find the following items after defeating the indicated opponent.

Opponent	Item
Archivist	Up to four cash bags (two usually left of the rock writing and two usually near the shrine behind Siris), small key (left of rock writing or during intermission sequence when Siris and Archivist are talking when the camera falls behind the wall; there is a very close small key there).
MX-Goliath	Five or six cash bags, sometimes one full health, and one large chest at far left of the room.
Stone Demon	Four cash bags (one behind shrine, one under bridge, one on the bridge close to shrine, one on ledge before the bridge), usually extra three cash bags in the small temple where Siris enters the room, and large treasure chest before the bridge.
Thane	Up to eight cash bags, one full health (on the stone ledges), one treasure chest (open after second kill), and medium key (behind chair left of Thane's chair).