Follow the tunnel leading out of the north end of the big room where you fought multiple enemies earlier. The steam has dissipated now so it's safe.
When you emerge in the room with the gun turret, use a rope arrow and rope ascender to open the reinforced door on the right.
Go through to an open area with an edible plant and another relic (3/3), a Portuguese Tin Coin.
In the corner near the closed door, on top of the stacked sandbags, is the Cliffside Bunker Treasure Map. If you've followed this walkthrough, it will be your last pickup so not very helpful. ;)
Move to the edge of the cliff, near the waterfall. Stand by the anchor post and shoot a rope arrow into the rough wall above the tunnel entrance ahead. Climb across and drop down. Use the Bunker Vista Day Camp if you need it. Then follow the tunnel back to Shipwreck Beach.