On this page of our game guide to Batman: Arkham Knight explains how to investigate the Clock Tower for clues to locate the Oracle. To get the clues you have to check the security system recordings and solve a small riddle.
After securing the area, head to the top of the Tower and unlock the way from inside. When you reach your destination, you will watch a reconstruction of the events. Look around the room and notice the knocked-over wheelchair. After talking to Gordon, use the computer located in the tower to watch the camera recordings.
Fast-forward to 33 seconds and zoom in on the top-right camera view. Scan the person that carries Oracle out of the building (shown on the picture above). You will learn that Arkham Knight is the one responsible for the abduction.
Stay on the top-right camera view and fast-forward to 39 seconds. Target the vehicle that you can see on the screen above. Scan the machine to confirm that Barbara is really inside.