Another theory which is ENTIRELY MOO:
That night, Brian and Clint want to score some cocaine to make the night a bit more exciting.
Clint arranged the drug deal/pickup and sent Brian to get the cocaine. The drug dealer, as a local, knew about the alleyway/construction zone that leads to the back of Wendys. That's why Brian exited via the construction door underneath the camera, because he didn't want to be seen picking up drugs. The drug dealer also didn't want to enter the complex at the escalators due to police being there. Who knows, the police may have even known who he was due to previous?!
So they agree the drug pickup in an area they know is mostly under construction, therefore off camera and Brian can enter and exit the complex without being seen to be picking up drugs. If the police suspected that of him, he'd have gotten in a whole world of trouble if they searched him.
When Brian didn't return, Clint knows that the drug dealer must've had something to do with it but he won't say. The drug dealer makes threats against his life if he tells the police ANYTHING. That's why he lawyers up and won't take a polygraph. Clint knows that Brian's disappearance is drug related and potentially a drug deal gone wrong.
I am interested to know also, would medical students like Brian and Clint be struck off from their universities for taking drugs? Surely any drug use in their profession would result in serious consequences?
There are no doubt holes that will be picked into with my theory, but the drug dealer could've even lured Brian into his car to get the drugs. Perhaps the dealer wanted more money, perhaps Brian wanted to pay less; but something went wrong and Brian was taken care of.
The dealer could've even told Clint that Brian 'ran his mouth' at him, something Clint would later say about Brian.
All just MOO but that's another theory for me that works.