We spent much of the past few days searching nearby bodies of suicides, and we finally found a Polaroid camera (even though it's 2011). We took a picture of the interior of the cave we've been hiding in and then set off in search of the door. When we got there, acting on advice that you guys had given us, we placed the photo into the triangles, which is apparently a photo frame.
Once we did so, we were able to open the door, which swung outward. When we opened the door, we saw ourselves looking in on our cave.
We shut the door, removed the photo, and found that the door had locked itself. We took off back to the cave before night fell, but found something rather unsettling when we reached it. Not far in, about halfway to the point where we had been sleeping, we began seeing blood smears on the wall. When we reached the point where we had been sleeping, we found the bodies of Brandon and Emily, two of the other survivors, disemboweled and propped up against the cave wall, as if sitting down casually next to each other. Despite the gruesome state of their bodies, their faces were untouched and appeared normal, with the exception of being ghostly white and frozen in a look of horror. Perhaps most unsettling, however, is the symbol drawn above their heads in (probably their own) blood.
We ran all night away from that cave, and we have found another one. We are sleeping in shifts to look out for anything so we don't meet the same fate.