A change in color.

Metallo sent a message to the Daily Planet describing that he kidnaped a bus full of people and if they wanted to see them alive and well Superman must confront him alone.
Going for the press was the fastest way to get his attention, gathering the materials for his plan took some time especially the most important component taken from Lex Luthor after a threat to sue him for turning him into an illegal weapon.

After some time waiting, the door to the storehouse that was used as a temporary hideout was torn open by Superman.
"Where are the hostages?" no beating around the bush the super-hero approached with the intent to finish this quickly.
"On the bus" Metallo responded pointing to the bus parked in the middle of the place in a nonchalant way making Superman confused.
"That's it? no ransom? no demands?" Superman couldn't find what's Corben's endgame here, but he needed to get the people to safety.
"Nope, and if you doubt me you can check inside" taking it as a challenge Superman stepped inside the vehicle to check on the passengers since the windows were tinted and any attempt to use his supervision were obscured by either the materials the bus was made from or the set-up of the room the vehicle was positioned, in any other day he would say this was a trap and carry the bus away, but the way everything was set up made him cautious "maybe there's a bomb inside that'll detonate as soon as it's moved or something like that" he thought as he stepped inside.