It is really stretching it for me to even contemplate that Brian is successfully evading what some poster enthusiastically named as the 'most expert law enforcement org in the world'...( the FBI ) in comparison to his life as is known up until then. He still lived at home . He didn't hold down a full time job, and when he did, he worked for Mum and Dad. He didn't own property, or even his own car. He had one girlfriend. He didn't play team sport of any kind, as far as I can tell.
Gabby is, as far as is known his first and only murder, and it is reasonable to assume, on that basis alone, that it was not tidy, or efficient. That the rangers who went in there could tell it was homicide from 40 paces away.
And then, he doesn't hide her body, ( the evidence of his crime ) , he leaves it in a well used area, and not only leaves her body, but then takes a van registered to her, and her bankcard , and then actually withdraws money from a bankcard that is not his..
Any 24 yr old would know he was leaving a trace on that act alone, using a dead persons card. He knows she is dead, because he has her van, and has departed the scene. Otherwise, he would be looking for her.
It is on this kind of serial repeated persistent cackhanded action of Brian's that I am skeptical of the whole idea of him doing this incredible evasive survival thingy on his own. That would take real instinctual cleverness and forethought, LOTS of forethought, a capacity to judge risk very well.