Speak to the Qunari standing near the Qunari Compound entrance during Act 2. A patrol was sent to the Wounded Coast but it never came back. Though the Qunari suspects Hawke of killing the patrol, nothing comes of it. You can then choose to search for the patrol even though the Qunari does not ask you to.
Head to the Wounded Coast. You will find the place where the Qunari bodies are lying. A group of shades and an abomination will attack the party. Defeat them and then report back to the Qunari at the Docks to complete the quest.