the-witcher-3-guide-witcher-contract-phantom-of-the-trade-route

After getting the quest, head to the designated map marker to examine the missing caravan.

Or, just head straight to the source of the missing caravan, which is through a cave, and which turns out to be Royal Wyvern. After killing this royal wyvern, two surviving men from the caravan meet and talk to you, telling you they ran and hid in the caves when the royal wyvern attacked them.

Head back to the Refugee who gave you the quest to receive your reward.