Someone will stand at four points in the city and wait for you to talk to him. Approach him, listen, then scan the symbol he leaves behind. Image is not mine.
1 will be available after you rescue Catwoman, and 4 won't be available until you save Vicki from the crashed helicopter.
Onnce you've met him four times, arrange the composite symbol as shown and head to 5. He'll talk some more, then leave. Easy sidequest.