: My bad, I figured it out. I guess I did know the first time.
: But what's up with that bob? Is there an unreachable hallway above me?
In early versions of the map, the BOB was behind a hidden door--he could be found (and killed) in the so-called Marathon 0.0 demo version of Arrival but not in the 1.2 / 4L demo or full release versions of the map... Much like the pillar-entombed ghosts.
For some reason he reappears in M1A1's version of Arrival, but the door, like the ghosts, doesn't seem to be there anymore.