Brian may be good at being on the run, as it turns out, but he sure does know how to draw attention to himself. I can't get a handle on why someone who had just killed their girlfriend would get in her van, drive it to his parent's house, and leave it sit in the driveway for days. He ended up using bank cards on her name for who knows what--why not just take a flight back to Florida? He could have easily ditched the van anywhere but in the driveway of his house. If he was planning to run, why not run west? His trail would have gone cold very quickly. If he was going to keep the van, why not just go ahead to Yellowstone and the other places they had already scheduled out? Well, I guess it's a good thing that he isn't a calculating type of guy because he could have made some different moves to avert the attention he drew at Spread Creek and back in Florida, and GP's body would probably never have been found.
This behavior is the major reason I still have a reasonable doubt of his being guilty. It would be way more than a reasonable doubt if not for the silence once it was known GL was missing. I can imagine only one reasonable reason why you wouldn't want to help find her or her remains. That it would implicate you in something. But maybe there is one I can't imagine. It would have to be very compelling though. Even his being missing doesn't have to be a sign of guilt of murder if it turns out he suicided. That could be remorse for not taking care of her and keeping her from whatever befell her after a potential parting between the 2.