From where you spawn in Patrol, go to the right. There will be a round building on the edge of a roundish area with Cabal fighting Vex. Look around the building, the connected hallways, down the ramps in said hallway, then go around the edge of the area to check on the rock spire and cave. After that, go past the spire, over the path, to another Cabal area with a huge doorway. Relic iron can be found around the rocks, in the round building and it's nearby cave, and to the right of the huge door. Rinse and repeat.