This achievement is earned during the first case Pray and Tell
While searching Hursts house for clues press RB to enter detective mode. There is a bucket of oil near the fireplace. Interact and this unlocks dialogue with the boy. (I noticed this on second visit to theHurst house, unsure if available on first search)
Fast travel back to Baker Street and Interact with Toby
Head back to Hurst house after interacting with Toby. You now take over as the dog.
You are tasked with following a scent of oil to locate a firearm.
As Toby walk near the fireplace to pick up the scent (look for a green trail).
Follow the trail outside into Hurst's back garden. Exit the garden, following the scent left up the street. The scent branches off into various dead ends. Just follow the cobble street near to a set of locked metal gates. Turn left and look at the large brown fence, there will be a gate handle. Interact with the handle and walk through the garden until you reach some downward steps.
The Achievement will pop as soon as you interact with the marker at the steps. You will switch back to Sherlock to search to house which has a photograph of Tom.