For me, an accident and foul play (including an accident that was deliberately concealed) are both viable options.
Suicide seems unlikely only because most suicide victims are found. However, suicide is the 4th leading cause of death among 25–44-year-olds
, so it's tough to write it off altogether.
I will not rule out the possibility that Brian is still in the building, somehow buried or entombed via misadventure. For the police to say that he isn't amounts to them claiming that they're too smart for there to be any scenario they haven't thought of—too smart not to have thought of every possible concealed cavity within a massive building that was being altered daily due to continuing construction—even though numerous people have disappeared in buildings and been found inside those building years later despite being missed during initial, supposedly thorough searches.
Foul play is a good possibility, too, and that broad label encompasses many different possibilities. Brian could have mouthed off to someone while walking home and gotten himself killed. He could have been mugged and killed while resisting. He could have gone to someone's house to use drugs and OD'd. He could have been hit by a car and then hidden. You name it.
The one scenario that can be ruled out, in my opinion, is Brian's having run off to start a new life. People have done that, but virtually all of the ones who do are running from the law, trying to escape a bad marriage, or trying to pull an insurance scam. The small percentage who don't fall into one of those categories usually have a severe mental disorder such as schizophrenia or dementia. For a 27-year-old single person with no outstanding warrants and no major mental illness to do that would be unheard of. The likelihood is on par with alien abduction, in my opinion.