An Icarus

Missing, extremely new, or very obvious to obtain.

Probably a reference to Icarus, a Greek boy who flew too close to the sun, causing the wax on his wings to melt and making him fall into the sea and drown. As a result, Icarus has become a cautionary tale about how dangerous one's own hubris, recklessness, and overreaching ambition are. Perhaps it's the Trailblazer's own hubris that they listen in on Welt and Acheron's conversation.