It is etched into the marble Carl parks his cart on, side-on, and he would never have seen it a day earlier.
The logo turns up underfoot. When the crawlers shove The Def Leppard Cart into the middle of station 36's main chamber and stop it between the ring of inactive stairwells, Carl notices a faint etching on the circle of marble: a side view of the Syndicate logo — the lopsided, connected, coil-like form rather than the flat rings. He says outright that he would never have noticed it if Mordecai had not shown them the night before.
It marks precisely the section of floor that falls away when the The Emergency Escape Tunnels open, and the portal Carl hangs over it turns out to be almost exactly the same size as the circle.
A faint side-view etching of interlocking rings in a marble floor.
The mark on the floor showing exactly where it was going to open.