Hang in there, Sandy. I'm sure you're taking nothing for granted, and not caving to her claims of "You don't trust me!"
I like Dr. Laura's advice:
Let them know that nothing in your house belongs to them, including so-called "privacy," and that they are guests in your home until they are 18, at which time they are free to earn their own whatever.
So far she hasn't had the guts to really screw up, but she's laying the groundwork. We're hopeful she'll get a clue before she does something life-changing, in the meantime we exercise constant parental vigilence.
One funny note: yesterday my husband was driving her and her friend around and the friend remarked that her mother was "in her business". My husband inserted himself into their conversation and informed them both that as long as they were minors, everything was mom's "business". They were quiet after that. ;-)