I don't think his band mates or his girlfriend know a thing. I just saw this case on Disappeared as well. With respect, I think the female investigator was very odd, and came to assumptions, or felt things that absolutely had no basis in reality.
She found it alarming that his fiancee didn't wait days to report Brian missing? I don't find that odd at all. Brian's car was left behind with his keys, his cell phone, his wallet, and $800.00 in it. Due to that and the fact that Brian was due back after his appointment but did not return, that his bandmates did not know where he was, and that he had been acting slightly strange, it makes perfect sense that she would not wait to call the police.
And that bit about her flirting? That was what he told his bandmates. It is not necessarily fact. I think he may have had an onset of psychosis of some type. If he has schizophrenia, 25 is in the age range when it usually manifests itself. Hence, he suddenly became suspicious about his girlfriend, suddenly needed to quit the band, and saw angels appear that told him to join the fight between good and evil.
Also, all of his bandmates and his girlfriend passed polygraphs and I did not see a shred of evidence that something bad was done to him - just the wild imagination of the investigator who seemed very odd to me - nothing else. The bent screendoor? I have one of those myself. There could be any number of non-ominous reasons for it.
I believe this young man could have wandered into the forest and eventually died of exposure, threw himself into a river, or taken off on foot and wandered as his bandmates say he talked about. I really can easily see Brian taking off, jumping trains and losing himself in some city as many mentally ill homeless people do.
I think the best bet to finding him would be to mail his flyer to as many homeless shelters as possible around the country, mostly in major cities. If he is alive, which is a very good possibility, he's on the street somewhere and that someone may recognize him.
Finally, altering his photo a bit to add scraggly, long hair and possibly facial hair, and sending that to homeless shelters, may help. Mentally ill street people can be quite resourceful and do come into contact with such agencies on a regular basis.