IMO, I don't think it was a gang member. I mean, wouldn't it be easier to abduct someone off the street in a poor neighborhood?
The area Sierra was taken from is a middle class area(?), with pretty expensive houses. I just don't see why a gang member would risk being seen with a car that stands out, and some houses have surveillence cameras. And it just doesn't fit in with the folded clothes, bag and the cellphone. It's too... organised.
I think that it's a psycho, maybe someone who have been stalking her. Someone who doesn't think about the consequences, but "needs" to take Sierra. The car really bothers me though, because it really stands out, and the police could've put up roadblocks if a witness had seen Sierra getting abducted. Why would someone risk that...
Just a theory. I have a feeling that this case will be solved soon. I just hope that she's alive.