The one in the southwestern corner of the small room across the lava does not spawn. You get the arrows showing you the exact spot, but when you use the Light of Deldrimor there, it shows up empty (the "no chest" animation).
When you step back a few paces, the arrows spoint you into the same direction again towards the exact same spot. But going there again and using the LoD results in the "no chest" animation once more.
My GW2 content charts: (1) LWS 4, (2) PoF , (3) Personal story (pre-Claw Island/Orr) , (4) LWS 2 , (5) HoT, (6) Orr campaign , (7) LWS 3