Batgirl spotted Robin's colors across the street, and she was kind of surprised Stephanie was still wearing the Robin costume. In the process she noticed that Doug -- in his Cypher costume -- and Spider-Man were talking next to that unused (and probably still unauthorized) spider-signal.
She watched the body language of all three as she crept up on Stephanie. Spider-Man had just stumbled onto some grand revelation, Cypher was embarrassed he'd just let something slip, and Stephanie was scandalized at what she was hearing through the microphone.
Then Stephanie shouted about Batman, nearly deafening Batgirl, who was peering over Stephanie's shoulder. She covered her ears and stumbled backward a bit, startled.