They reached the bottom of the stairs and found the tunnel branching in a t-junction; left, right, or forward. Calls on the radio revealed nothing. The female NPC was, by this point, thoroughly around the bend, clutching her prybar and crying to herself, begging the others to just take her home. They just ignored her now, and she meekly followed where ever they went, afraid to be left behind.
There was a strange, almost mechanical rumble and the floor vibrated, sand falling from the ceiling, but the tunnel remained intact. The incident coincided with a burst of static over the radio before the usual ambient spooky shit continued to haunt them.
Now another player, not one of the two who thought they'd seen movement earlier, was seeing things in the corners of his eyes, not finding anything when he turned to look. The players, sitting in their chairs and hissing through their gasmasks, were obviously experiencing the same thing as they sat in the flicking candlelight. Excellent.
They chose to split into three groups going in each direction down the tunnel (one NPC in each group, with the guy who vanished and came back in the group with two players in case he tried something) and they would walk fifty feet down each tunnel and then return to the stairs, reasoning that if their lost member had seen the stairs from where he had been laying one group would find him if they had descended the same stairs.