I don't have a teenager yet, but DD is 10 (and often behaves like one!) so it obviously is something that we talk about a lot, as it will creep up on us before we know it.
I recently found out (won't go into how) that, when I was a teenager, my parents routinely used to go through the stuff in my room and "check up" on me.
Not sure exactly at what age they did this, nor how long it went on for. All I know is that everything must have been put back exactly as it was, because I was quite a meticulous, tidy teenager... I'm amazed I didn't notice.
I don't think they were looking for embarrassing diaries or jazz-mags. One of their friends had a son who took his own life at around the same time (I only vaguely remember about this, to be honest) and he had concealed a gun from them. I think they were just reassuring themselves I wasn't keeping anything similar from them. It makes me feel a bit 
Maybe this seems a normal precaution to those of you with teens... Or does it seem like a huge invasion of privacy?
The person from whom I found out about this says we absolutely must do it with our children and that deceiving your child in this way is a small price to pay for their safety. I'd like to think I wouldn't have to, and that we'd know to look for other signs if one of our children was having problems... but what do I know?... Teenagers are a whole new world.