I'm with Occam on this one; except in truly rare instances, the most likely scenario is usually the least complicated. It still sounds like Brian left through the band exit, even after the band had left while he was with Amber and Brighton. There would've still been people leaving that way for the after-party, going to the bathroom etc on the way out. As long as they had the address/ vicinity of the party there wouldn't necessarily be a one-time herd leaving before 2:30am. With a school that size, and presumambly not the most sober of groups, he wouldn't stand out.
I think Brian made his way to the Wendy's lot for a ride to the party, for drugs, or a hookup (male or female). He would've intentionally avoided Clint and Meredith if he either didn't want them to know his destination, or simply didn't want to go home yet and run risk of his girlfriend wondering why he told C and M to leave without him since he would've had to come up with something if he'd run into them. He would have assumed he could tell them he just lost them in the shuffle the next day. If Clint were hiding something at the time, it could be as simple as Brian did drugs and/or was cheating on his girlfriend. Both of which would have been a big deal at the time to blow your friend's cover. Clint may very well have released that info later, and police haven't disclosed it.
After the Wendy's rendevous, he may have had a fatal drug overdose, alcohol poisoning, or drunken fall and then dumped in the woods, possibly just to sleep it off. If he wasn't dead yet, he would be of hypothermia and just hasn't been found. A darker theory would be one/both of the significant others of girls at the bar, or anyone he may have run his mouth at for that matter, who followed him to Wendy's or the party. Or if they weren't at UTS, were called to show up. Less likely are being hit by a driver who panicked and was able to get his body out of the road without being seen or a mugging/abduction/murder/disposal by a stranger or strangers.
While it's possible he ran away to start a new life, even possible though unlikely that some in his life new his desire but not the exact plan, it seems highly improbable without some serious planning and resources. And certainly not after a night of drinking. If he made it home and planned to leave the next day it'd still be unlikely for him to pull it off without help.
As for the construction site, I don't think his body is there. If he'd been bonked over the head with rebar or somehow died accidently someone would've had to find him and then decide that whether accident or homicide it wasn't good public relations for UTS or OSU, whether or not that person was involved. He/she would then get his body out of there asap, even if it had to be hidden until the next day. That someone would need access at off hours or to have gotten his body to a dumpster after even staff had left. It would make no sense to try to permanently hide his body on the premises, he/she would most likely assume in a college town that the deceased's friends would lead police right back to UTS. In my estimation, the only way his body could be on premises is the farthest of flung scenarios where his body made it's way deep into the ground by itself ie sinking, and sinking below the level of the actual construction itself, which is unlikely. I don't think the zoning would be allowed over that kind of water table. And he can't be just behind a wall or a floorboard. If he died or was killed at UTS, it was an awfully short timeline and he's probably in a landfill, which could have been traced in those first few days.