She isn't trying to hide stuff very well is she? Does she want to be caught? Maybe, I don't know.

I would try and completely change tactics - she is expecting the 'talking to', the dissapointment, the lectures.

Whatever you are doing, turn it on it's head. Hold her close, go into her room later on in the night and speak to her like you would MY dd, not your own.
Write her a letter telling her the things you like best about her.
Go to her school and get her called into the office before lunch and run away with her for the afternoon. Get your eyebrows waxed together, take her to a cosmetics counter for a makeup session, or for a pedicure. Recognise she is a young lady and treat her like one - even if she doesn't deserve it right now. Give her something better to aspire to than 'misunderstood teen' (oh how they love that tag!).

She's a new butterfly right now - she is testing her wings out. Oh lord they (their behaviour) can be ugly sometimes - shake it up, change what you are doing, anything to get her out of the current mindset. She is your very first girl, remind her of that xoxoxo