Prey Mooncrash — the standalone expansion — uses a roguelite structure where the moon base resets with varying configurations. Five characters must escape across five escape routes, and the game’s final state requires all five to escape in a single run. Most players understand this as the completion condition.
The fifth ending requires something the game does not advertise: approaching the main escape route using a specific character build sequence that generates a facility state where a sixth character model — unplayable, unnamed, present in the facility’s employee manifests — briefly appears and can be interacted with for a single dialogue exchange before the run ends.
The sixth character describes the moon base before the outbreak: competent, functional, full of people who thought they were doing important work. They name two colleagues by surname. Both surnames appear in the casualty lists in Prey’s base game — Talos 1 personnel who died before the player character’s story began.
Arkane connected the expansion’s hidden interaction to the base game’s casualty records. The sixth character’s colleagues are dead in the story’s timeline, and Mooncrash is set before those deaths. It is a prequel moment tucked inside a roguelite’s failure state.

I’ve put 200 hours into this game and never caught this. The developers really reward the obsessive players.
Currently on my first run and now I have to go back and look for this. Worth the detour.