This quest/content is included in the Signature Edition of Dragon Age 2. It will be automatically unlocked at the beginning of The Destruction of Lothering quest although its only possible to complete this in Act 1 onward.

To complete this quest during Act 1, you have to go to your Uncle Gamlen's Home in Lowtown  and enter the southern room. During Act 2 and 3 you can find it in the northern room in Hawkes Estate in Hightown.  From there you can find a tome named Of Things Not Lost inside the chest marked as Special Deliveries. Upon taking the tome, you'll complete this quest and receive a codex entry Of things Not Lost and 3 enchantment runes. Rune of Fire, Rune of Impact and Rune of Protection.