There's a reason vigilantes don't use elevators and...okay well this isn't it.
Robin had gone down to Star City to look into the weapons dealing business that was connected to the trade in Gotham. A few nights should be enough to pick up what information he needed. In fact, he was in the middle of swinging through town after rattling one of the smaller fish when an alert tells him the alarm's been set off at a building not far from where he was. 

Well, crimefighting called. So Robin swings that way and enters through a window before blending into the shadows again.