On the top of the Gotham Bank building, Batman gets a criminal and wraps him on his rope. Then he holds the criminal over the ledge, making him see the ground. As the man yells in fear, a Parademon smells it and tries to eat him. When it comes, Batman hails the person back up and captures one.
Batman traps the Parademon in a net, but the creature explodes, creating symbols of three boxes.
"It's 'cause they know he's dead, right? Superman. He's gone. Where does that leave us?"
After the Parademon exploded, Batman and Alfred recognize the three box symbols that were on Luthor's notes. Batman then decided to assemble the team of four heroes.
While Batman moves and before he leaps off the Gotham Bank rooftop, the Burglar questioned him about Superman's death and asked where does that leave us; that leaves Batman to believe an invasion is coming.
Bruce learned from Amanda Waller's files that Arthur Curry visited a village in Iceland (as a mysterious "stranger from the sea") during the winter, bringing fish to feed the hungry, always coming on the king tide. On the day right after the king tide, Bruce arrived at the village and asked to speak with Arthur.