Yes, it is possible she lied and told her parents she was spending the night at her girlfriend’s house. Most parents would be driving her there and talking with the girlfriend’s parents about the arrangements.
There is no excuse for a high school student camping out overnight with her boyfriend.
I'm not making excuses...just being realistic. Teenagers are going to do things they don't want their parents to know about. And parents have to trust their kids somewhere along the line. My parents trusted me by the age of 17. Did I do things I didn't want them to know? Yes. But I never did anything really bad...because I didn't want to disappoint or embarrass them...we lived in a really small town.
My friends and I smoked...kept them in a coffee can under a tree on a dirt road. That was our biggy risque thing. We also had a 10 o'clock curfew. We were "normal" back then...would probably be social goody-goody misfits today.