There has never been any evidence of foul play. He was a low-risk victim, and theorists have to resort to elaborate scenarios for him to be murdered. I've seen all kinds of suggestions that he witnessed a drug deal and was silenced, or he overdosed with the band members and they dumped his body in a waste compactor. It doesn't sound remotely believable. The idea that he voluntarily disappeared in the middle of the night, under the influence, without any credible sightings is equally dubious.
Occam's razor must apply. We have a man occupying an area that had construction work going on, who was intoxicated and may have tried to leave through a back exit. He stumbles and falls into a blind spot, perhaps he suffers a concussion and falls unconscious. When construction resumes the next day, Brian's remains are unknowingly concreted over. Until some new evidence comes to light, or this area is dug up (unlikely this will happen for some time), that's the theory I'm sticking with.