In Act 1, Jansen is involved in the quest The Bone Pit (quest). After clearing out the mine, Hawke will stumble upon a survivor, Jansen, who warns him there is a large dragon. After slaying the dragon, Hubert asks you to find his surviving workers. During the quest Get Back to Work, Hawke has to convince them to return to the mine. When you travel to Lowtown, you'll find Jansen and the workers near The Hanged Man.
In Act 2 you'll find Jansen in the mine. He alerts Hawke to continual problems in the Bone Pit, leading to the following quests:
After completing Pick Up Pickaxes if you approach Jansen he will complain about the smell in the lower mines, the sudden lack of animals in the surrounding areas and that two of his friends went one missing, one of which was found "half eaten and with big claw marks". After this 3 lines he will only say "I always feel safer with you here boss". These plus the growling noises that can be heard in the background point to the High Dragon that Hawke meets there in Act 3.In Act 3, Hubert once again asks Hawke to investigate the Bone Pit after a caravan was completely destroyed, with all killed save a horse. During the quest Mine Massacre Hawke discovers that a High Dragon has once again made a nest in the Bone Pit and drives it out. It's presumed Jansen was killed when the dragon attacked.