Recently my mother and I were chit chatting and she told me about the things she found when going through my bedroom (like I didn't already know). She said that she wished she hadn't been so noisy because in the end it hurt her more than it did me. Yes, she did find out some things that I wasn't supposed to be doing and prevented me from doing them. For example, she told me she found a note from my friend plotting a weekend away with boys but telling my mother I was staying at her house, she didn't let me go away that weekend. My mother said that for months after that my notes always included something about hating her for ruining my life. She said it broke her heart every time she read it, but she still couldn't stop herself.
As for the smoking, are there sixteen year olds who don't smoke? I know I smoked in high school, but once I got to college it wasn't cool anymore so I quit. Just make sure she knows that smoking is bad for your health, illegal for minors, and that you hate it. Honestly, how are you going to prevent her from smoking? There's always a time and place away from parents to do the things they tell you not to do. If you make a big deal about it she's going to know that you invaded her privacy and broke her trust by going through her bedroom and that's going to be bad news.