A few thoughts, for what it's worth.
I think, for whatever reason, the perp did see Sierra on his daily drive to who knows where. Maybe he was the one she referred to in a tweet about someone in a car. ? Although, this crime may be considered random in that he didn't "know" her personally, he knew her routine and that she was the only one on that road at that bus stop. I do no believe that March 16 was the first time he saw her and then he acted.
The news report that broke the news about the skin being the evidence on her clothes also noted that the Sheriff's Office wanted to do an arrest sooner and they were battling with the DA's Office, who wanted to hold out to make sure that they had a solid case before arresting him. I strongly believe that they did surveillance on him for so long, because they were seeing if he would lead them to Sierra, alive or dead. Plus, I think they moved once some type of forensic testing came back proving that she was dead. Maybe they really were working it as he may have her somewhere and if we arrest him, he can't lead us to her. But, all of that changed once the newest test results came up giving them what they need to not only prove that she was in his car, but that she is deceased. Add to that, the new found knowledge that connected him to one previous assault attempt, and they decided to move. Him being arrested at gunpoint in the Safeway parking lot is very interesting indeed. LE has obviously done an amazing job so far, including being private all the while trailing this guy, searching his house and impounding his car.
I also believe that he used some form of device to subdue SL and after taking her, he tossed her phone. I bet his DNA or fingerprints are also on her phone. I think he took her far away at that point. I think (for whatever reason, I just can not still understand) he brought her bag back (maybe at night) and left it by the shed. I think that some sort of evidence of water was on her clothes or on the bag and that is what prompted the water searches. I do not think he harmed her anywhere near where the bag was found, unless he was on some type of drug. It was broad daylight and for him to actually succeed in kidnapping her, his adrenaline must have been on high and he would want to get far away from her home.
Also, if Sierra was actually put in the car, it seems there would be at least one video shot somewhere that shows Sierra in the car. Although, to me, it seems he would have put her in the trunk, so she couldn't be seen by anyone while traveling. Although, if she was in the trunk, not sure how he would have her phone in the front to throw out the window.
Lastly, it was interesting when the Sheriff was asked by a reporter at the press conference, if he has given an alibi for the time Sierra was taken. She kinda stumbled and then something about him not really giving them any information yet... If they suspected him and impounded his car, I can guarantee that one of the first questions he was asked is "where were you on the morning of March 16, as well as the rest of the day?" Right?
Wishing everyone peace in their sadness.