"Retrieve the Stolen Chest" is an objective in the mission Johnson's Errand
You find the Stolen Chest inside the fort after Shooting the Powder Kegs
Dialog[edit]
Mercenaries: Fall Back! Fall back! We'll be safe inside
Charles: What now?
Thomas: We can blow the door with those. Go on. Shoot'em... Guess they wasn't so safe inside after all... Bet this'll fetch a pretty penny
Haytham: Save your looting for AFTER we've found Johnson's Research.
Mercenary: Lay down your weapons and I'll consider letting you live
Haytham: I make you the same offer. We've no quarrel. I only wish to return this chest to its rightful owner.
Mercenary: Nothing rightful about Mister Johnson
Haytham: I won't ask again.
Mercenary: Agreed.