I suppose we can only speculate as to if brian put something in brightans purse as the video is unclear, though it does look like he has his phone in his hand atleast at some point (given his head movement).
However, strangely one of the guys in front of brian and co, seems to put or take something from one of the other guys pocket just a second later? Which seems a strange coincidence (see attached).
Also do we have any idea where the random girl coming up the escalator was going? She turned the opposite way to UTS yet there was nothing on the other side except the door to a construction site and of course brian/amber/Brightan? Do we know who this girl even is?
Also, we know as per Amber that you could indeed open the construction site door as she had previously used the room to call her boyfriend on a different night at UTS.
Brian said he wanted to walk the girls back to their car, but they'd already left while Brian (presumably) stayed back to chat to the band members (as per Clint and meredith). IMO Brian probably tried to catch up with the girls after they left by taking the construction site exit, forgive me if I'm wrong but I'm 90% sure this exit takes you out on the side of the car park, where the girls would have been parked. Brian probably knew this exit was on the car park side and used it to try catch up faster with the girls.
Whatever happened to him after walking through that door is anyone's guess. I'm personally not sold on the idea he made it as far as Wendys (given that there's no cctv footage of him). I could be being naive here, but couldn't Brian have been at wendys earlier on in the day? Or even the day before? And that's why the dogs picked up his scent there? The dogs were also interested in the construction site, which would support my theory of him atleast being, but not necessarily dying there.
Assuming he did make it out of UTS alive, I know alot of people will say the chances of the same person being attacked by a serial killer, or falling into a river, being the same person that just so happened to be the only person not seen leaving UTS that night, are very low. Personally, given that we know brian was drunk and had a tendency to mouth off at people (as per Clint and also Brian's MySpace bio which reads "PISSS OFF YA ******") I think its entirely possible he ran into trouble outside of UTS, perhaps one Smiley Face Killer.
I think that whatever it was that Brian was doing with his phone (as seen in the uncropped video), it resulted in his phone being turned off, for whatever reason. I believe i'm right in saying, his phone was then turned back on about 10 minutes later, could this have been an unsuccessful attempt to try and call the police, whilst being attacked?
On a side note, i did stumble across what is presumably a youtube troll, who was posting comments on videos about brian, saying he knew what happened to him. Usually, i'd dismiss this as a typical internet troll, however i've never seen a troll with a video as creepy as this one on their channel: