I think you're over thinking it, because I over think and turn something into a bigger deal than it is myself!

I tried smoking when I was about 15 and hated it and was against it as my parents are smokers. I also snuck out a few times in the middle of the night to meet with friends. Smoking on their part was sometimes involved but it wasn't why we snuck out in the small hours to meet. It was about spending time with friends, being a teenager and doing something you know you shouldn't be doing (sneaking out, not smoking in this context). Often, it was because there was a lot going through our minds and we needed someone to talk to and it was always easier to speak in person and it was always during the night that our thoughts were going crazy. About school, about parents, siblings, friends, boyfriends, girlfriends and so on...

I understand where you are coming from, but if you don't trust that she doesn't smoke when she told you she doesn't (especially from the perspective of my teenage years) then you really are perhaps over thinking it. As much as you love her and care for her, life is too short to worry about things like this! I was the kind of girl who found it hard to say no to things and gave in to peer pressure, however, smoking wasn't one of them. I didn't like it and I always said no.

Hope this helps somehow