Just watched Bri's episode of Disappeared, so heart breaking. My theories I drew up while watching:
1) She was going to meet someone after work. Would explain her being upset earlier when she was shopping with her mom, and left to meet someone for a little bit. Possibly drug related? I do feel the whole drug angle may make the most sense, although I don't believe she was into drugs hard core, I remember they said she tried a few things outta curiosity. The fact that she also is linked to two drug dealers is what makes this angle make the most sense. Could someone have set her up?
2) The show mentioned she often picked up hitch hikers. Could this have been the situation that turned bad?
My heart goes out to this family...praying for you, and keeping Brianna in my prayers as well, what a beautiful, good-hearted person.