I have tried to go back multiple times to try and finish this mystery. Enemies spawn each time that I go back and the NPC completes his dialogue but doesn't give me an option to speak to him. I have wandered around the area multiple times but to no avail. I have stayed hidden whilst he finishes his dialogue but still no option to speak to him. I have the latest patch (1.6.1) which should've fixed the bug a long time ago. I looted everything when I first tried to do this mystery but have not been able to finish it since.