Carl's locator skill, used to pinpoint the unmoving Louis inside the Reaver castle's shifting maze.
When Doctor Bones produced Doctor Bones' Paper Map of The Reaver Castle from a previous infiltration, Carl activated Find Crawler and located Louis immediately, still unmoving. He could not tell what the wall configuration looked like around him, but he pointed to the spot: an area of line cubicles, meaning that if the configuration matched the old map, Louis was in a room no bigger than a coffin. He then marked Samantha's position just on the other side of the wall, giving Juice Box her extraction route.
The skill that fixed Louis's exact position for the rescue.
Also appears in books you've already read — spoiler-safe to their endings: