The hidden-signage side of Carl's map skill reads a portal placard nobody else in the yard could see.
Escape Plan's quiet second function — access to the dungeon's own concealed signposting, the Dungeon Locator system that intelligent mobs navigate by — turns out to extend to industrial infrastructure. In Station 75's cart yard, the small sign above the first of the five track portals can only be read using the skill, and what it says is a single word: "Trainyard." That word, plus a screenshot through the portal, is how Carl identifies Trainyard C and its two ogre-sized ghouls.
It is a small use of the skill with a disproportionate payoff, since the difference between the yard's five portals is the difference between a one-way trip to a rail yard and a dialled run down any of nearly two hundred coloured lines. The four remaining portals post their destinations openly as numbered colour lists; only the unremarkable one keeps its label hidden, and only Carl could read it.
No visible form; hidden placards and mob signposting made legible to one crawler.
The skill that reads the station's hidden portal signage and identifies where its plainest doorway leads.