Shannon's mother told authorities Shannon had 2 phones, a regular use phone and a burner phone for clients.
http://www.foxnews.com/us/2011/04/09/police-second-man-partying-missing-new-york-prostitute.html
It is possible Shannon was there that length of time to mess around with Brewer first, (not into necrophilia) and the 2nd man who was staying came up stairs for the killing. Brewer mentioned she went to the bathroom and came back all crazy acting. Shannon may have overheard the two men speaking, and feeling already woozy from the night of partying, overhearing or seeing some interaction that led her to
know with certain they were going to kill her, she freaked out.
I am thinking they confiscated her phone that was possibly laying out on a table, but surprise she already had a call en route to police on her regular phone. (They found one phone. Where was the other) She came out from bathroom already on phone with police. Their killing plan ruined, so Brewer called for her driver to come get her.
She may have picked up on an eye clue or other clue the driver was in on it. And she bolted not trusting anyone. The other girls lie so close and how nice for him to have a great spot a few miles down. The 4 may only be his kills and he buried them with opportunity with no knowledge another killer like the same desolate spot with the dismemberments.
Shannan was not a dumb girl, she was an honor roll student with a quick wit. She knew and could pick up on what was being planned.