Near to the 'giant master chief helmet', just before the pillar of rock where you fight the tank. if you go to the far left hand side and climb to the top of the rocks which you do not need to jump to get up. in the very far corner, you fall inside the rock and can move around without being shot or being able to shoot out of the rock. You are able to get back out if you go to the same spot where you can fall into it, however I thought it might be worth letting you guys know, there seems to be this issue.
If you would like to know exactly which rock I cannot find a way to screenshot from my PS3 and point it out, so if you find me on PS3 destiny I would gladly show. just search for "KrayZ-t" on PSN or in game in Destiny on PS3