Just as Scarlett is preparing to leave San Pasquale with Don and Nesto, as she is about to enter the tunnel to the gorge, a villager, Casimir, will be standing by that tunnel entrance. As she approaches for the first time, he steps forward to warn her, and makes a request.
Poor Rupert was the gatekeeper at the tunnel through the gorge. It's likely he was killed, either by Assassins or the creature now keeping anyone from passing through the tunnel (a Lector). His relatives request that if Scarlett should find Rupert's body, she returned the necklace he always wore to them as a family heirloom.
In the gorge tunnel leading from San Pasquale to Venice, Scarlett will see Rupert's body being eaten by some nasty demonic creature, a Lector. After she has dealt with the two Lectors in the tunnel, if she searches Rupert's body she will find the necklace.
She should return to Casimir at the entrance to the tunnel to complete the quest. (She can choose to give him the necklace, or keep it for herself.)
She can also grab the necklace without killing the Lectors although the subsequent dialogue with Casimir does not change and asserts that she has killed the Lectors.