"Someone's missing…"
During the commotion caused by the chamber's discovery, the fifth member of the group had strangely disappeared. Even though Leo's eyes didn't leave him for a second, the darkness caused by the torches' concentration on the wall allowed him to quickly disappear from Leo's sight.
'As expected, something is wrong with him.'
Not every single event in a novel's world was necessarily described in the novel itself. The only reason Leo came to learn about this expedition as a reader, was because something like Jared's death was important in relation to Ray through the second heroine.
Confusingly enough, Jade's brother had met his death during the exploration of a Sword Anomaly which collapsed as a whole in the middle of the expedition.