In my opinion Michelle's disappearance appears to be a more personal crime (ie was perpetrated by some one who knew her), rather than a stranger/random abduction.
It occurred in a very narrow timeline. Statistically, random abductions are rare, so for it to happen in such a small timeframe, never mind at all, is unlikely.
The phone being discarded, intact, in a separate location. I would think most strangers would not take the time to do this since there is very little about the victim, herself, besides any evidence on the body, to ID the perp. So, why attempt to distance the phone (which could be used to ID the victim) from the actual victim? I would think only someone who knew the victim would do this, without even knowing he was making a BIG mistake.
The removal of the Glow decals. I do not believe a stranger would take the time to do this. If the stranger even took the time to attempt to disguise the vehicle, they would simply back the Hummer into a parking spot to delay a witness from IDing the vehicle. Removing the decals is the act of someone who knew Michelle and knew the connection Michelle had to those decals (HER business).
Replying to the brother's text was not the act of a random attacker. First off, why even reply? To delay. A random attacker would have no need to do this. Also, I do not feel a random attacker would feel confident enough to reply to a text without knowing the relationship his victim has with the person ending the text.
Taking the keys from the Hummer after parking it. I think that a random perp would just leave the keys behind. Taking them shows a subconscious desire to protect the vehicle from theft. This would be odd for a random attacker. Why else would you pocket the keys? You clearly can't keep the keys for yourself (they will always be a direct link to whoever parked that Hummer), but you do not want anyone else driving off with that vehicle, so you take the keys with you. I don't think that the person who parked the Hummer even realized they were doing this, it was just second nature. But a random attacker wouldn't care about the vehicle at all; he got what he wanted...Even if the keys are found discarded at another site, this will hold..
So, those are just a few of my thoughts on why this wasn't a random attack. Still thinking on reason/details that would make me lean toward a random attack. I haven't come up with any yet, but would be open to suggestions...
Not saying these details lead to DS being the perp, but they IMO do point to someone who knew Michelle., which narrows the suspect pool considerably.