I think that Sierra left her home and left with someone that she'd known. Comfortable around them to get into their car which explains why the dogs had lost her scent at the end of the driveway. Then, the story could go a series of ways. They could have driven in the path that the following: making a right to at the intersection, by making that right they could snatch her phone upon reaching the intersection and throwing it out of the drivers side window. That is plausible. They could've THEN made a right at the site of the bag but not tossing her bag out of the window. By now, Sierra could've been frightened and urinated a bit. Hell, everyone does to some extent when they are in an intense stage of fear. Take her to an undisclosed location and strip her. The rest of the path I wouldn't know. After they've "done whatever," they could've returned on the path that they've chosen, neatly fold her clothes, place them in the bag and toss it at random (bag site). That's just ONE theory. The other is that one of her family members could've been angry and frustrated with Sierra to the point where they'll be unstable enough to harm her. After keeping her against her will somewhere or doing the inevitable, they could've planted her phone and bag, returned home, file a police report and missing persons case, and the rest is just a "class act."