Sherlock Holmes: The Devil's Daughter

At one point during the ‘Prey Tell’ case, you return to the Hurst house to continue talk to Tom. Holmes notices a box beside Tom. Examine the box and you will find a key, a badge, oil and a ramrod for a rifle. While looking around the apartment for a rifle, using your detective mode, you see a bucket with thick liquid (oil) in it. Searching the bucket reveals a scrap of cloth with oil on it. You deduce that you need Toby, your dog, to help find the rifle.

After you conclude your search, return to Baker Street to retrieve Toby. Travel back to the Hurst house and you will take control of Toby. Following the green mist, make your way out of the apartment and house and follow the trail along the street. At two points, the trail will split. Just keep following the trail to the end and you will find steps that lead down to a door; a basement. Press to bark and Holmes will catch up to you. When you have control of Holmes again, the achievement will unlock.

This achievement starts at [39:30] in the case walkthrough video in “The Hunter Becomes the Prey.”