what about the other exits. I have never been there but they said that the escalator leads to other places too, like a movie theater>
I have doubts about the service exit. During an interview, a detective speculated that it would be very difficult to navigate the area and then squeeze through the chained up door (however, Brian does look slim and fit). Since search dogs didn't pick up a scent in the construction area and there was no blood evidence, I think the most likely point of egress is through the main exits.
Consider the possibility that:
1. He wasn't recognized on the video. The quality is never very good on surveillance video and altough the family and detectives reviewed the video (frame by frame, I'm sure), there is always a chance he wasn't noticed, or disguised in a way to make him unrecognizable.
OR
2. He left (or was forced to leave) in a "blind spot" outside of camera range (like along a wall). Most surveillance cameras are set up to catch the majority of activity in an area, there are often areas that they only catch the view of a foot or hand.
If you consider this, it is a HUGE coincidence that Brian would just "happen" to exit in such a way. An even more interesting theory is someone abducted him inside the bar and was aware enough of the cameras that they deliberately avoid the camera view when they left with him.
It's all speculation. I just wish we had some small indication of what happened that night.