Building 5. Search the surrounding buildings (both inside as well as on top) every once and a while whilst running building 5 over and over. You can get on top of BLDG5 by jumping through the hole in the roof. Obviously titans can just jump straight up, while warlocks and hunters have to jump atop the overturned wall or w/e it is. Chests can appear ALL over this particular 'zone' so keep a sharp eye and you'll have so much relic iron you actually trade it for rep; I know. Also, if the light behind the boxes in front of BLDG5 is lit when you arrive to this area, I've noticed chests NOT in buildings seem to respawn MUCH quicker; particularly the one RIGHT next to said light.