I've wondered this too.
This reminded me of my very first reaction to those early rumors about possible overdose and unnamed drug dealers... My earliest thoughts about what happened, and the phone call to DR were:
1) Lauren was in some kind of distress at this point. They knew she had either gotten drugs/ done drugs with DR earlier in the night, and called him to find out what she had done, or what to do... (Or possibly Lauren herself called, for the same reason)
2) Lauren had left with the intention of coming back, and when she didn't, they called him to see if he knew what happened or if she had ended up back at Smallwood. One thing that comes to mind, given the rumors, is meeting someone to pick up drugs. In my experience at parties in university, drug dealers did not come and drop off the party supplies at the door. Someone would have to go meet them, or drive around the block with them to make the exchange. Girls often did this, even by themselves. (Seems crazy now, but it was assumed at the time that the person was someone that people knew so it was okay) I wondered, early on, if it was possible that Lauren went to meet someone in that parking lot/ alley and never made it back... But then, it seems that if something like that happened, there would have been phone records leading to someone else, and possibly more cooperation from the POI. It also assumes she was in the condition to not only leave by herself but that she wanted to continue partying, which seems doubtful based on the other evidence. And that doubt brings me right back to questioning the stories of the POI... more thinking in circles.
(I have to add that I don't really like speculating about drugs because I see that her parents have been upset by these rumors and that they have caused some negative backlash against Lauren. At the same time, I've always felt that the early rumors were detailed enough and consistent enough that there may be at least a grain of truth in there somewhere... )