This leftover detonator reminds you of your initial victories in Dr. Graves's game...
The Hearts of Darkness - Rogues' Contest
It began with a summons from Dr. Graves, a grim surgeon with a short temper. He gathered you and four other Villains together and proposed a contest. The prize: unimaginable power and resources at the winner's disposal, courtesy of your unnamed benefactor.
You started by making sure that Arachnos took little notice of you. You played dumb for the Arbiters and got them fighting amongst themselves, and returned to Graves.
During the first round of the contest, you managed to trick Dollface out of finishing a race to the finish, and stole the win yourself. Graves, though displeased, was forced to concede the first victory to you.
For the second round, each contestant was required to formulate a plan of their own design, to display their villainous prowess. After investigating the others' plots, you came upon inspiration: you would destroy a Longbow base, lighting it up for all of Mercy to see!
As you engaged in your plan, a mysterious voice began to assist you. It directed you on the specifics of when and how to destroy the base.
As you were about to evacuate the base and let the explosives run their course, the voice revealed itself: Scirocco of Arachnos had assisted you! He directed you to continue participation in Graves's illicit contest, in the hopes of tracking down and capturing your unnamed benefactor.