She might have lied to them so they didn't know.
From seeing it first hand, some girls will tell mommy and daddy they are going over to their best friends house, and then go, leave and meet up with someone else.
Its problematic in NYC.
Or, even slicker, the boy friend is the 17 YO brother of her "best girlfriend", whom she meets up with at the girl friend's house.
It could also explain how she met the 17 YO in the first place.