Entrances!
When they got through to the welcoming center, Sgt. Joe-Bob asked the question Tobias was thinking: “Hey, where are all the bodies?” Because there weren’t any. No corpses or scattered debris, either. Somebody had cleaned up.
There were other clues of human habitation, starting with how the power plant had been restarted. The life support systems were also on, although according to Tobias’ suit the oxygen levels were uncomfortably low, and the temperature was just above freezing. From Tobias’s point of view, that in itself was good news: even if the life support system itself was damaged, poor air was still better than no air; and anybody that ended up being evacuated here could just wear warm clothing.
He wouldn’t take off his helmet, though. And Tobias wouldn’t let anybody else take off theirs, either. This site had been overrun by maniacs once, and right now it should be a vacuum-heavy tomb. That it wasn’t seemed like a bizarre miracle, and Tobias didn’t trust it for a second.