I think there is a lot of evidence to suggest that Shannan was disoriented and confused that night. We have Brewer claiming that she went into the bathroom, and when she emerged she was acting erradically. She was acting so strangely that her driver, Michael Pak, says he had to ask her what movie she had seen that evening in an effort determine her mental state. We have Colletti saying he tried to ask her questions, and she just stared at him and kept saying, "They're trying to kill me!". Colletti also recounts that when he told her police were coming, she fled the house, falling down his porch stairs. We have Det. Varrone saying that he heard the 911 call and Shannan sounded out of it. The 911 dispatcher tried to ask her questions, and she either couldn't or wouldn't respond to those questions. She didn't know where she was.
That's a lot of different people reporting the same thing. They aren't ALL lying or mistaken, imo. If we add in Pak's statement that Gilbert and Brewer got into the car and left the house for a brief period of time, and Shannan's call to CVS some time after they returned, it isn't hard to draw the conclusion that Shannan was in some kind of drug stupor when she fled the house. That's not an established fact, of course, but there's enough evidence which points to that for me to think it's a fairly reasonable conclusion.
All just MOO.