Oh Kim, how you described your neighbourhood, that is scary. Why oh why do SOME teenagers think they are invincible.
I use to hear my sons most of the time say, "Don't worry Mom, I can take care of myself." Yeah, sure. I'm sure that 17 year boy that was attacked in the metro by 5 guys last week, told his Mom the same thing.

Obviously she has her reasons for doing this and heaven knows if you will ever find out what that reason is. I hope and pray for her own sake and for yours, that this is a one time thing and she has learnt her lesson.
You did the right thing by calling around to find her and by grounding her for it. I hope you and the hubby have a good sit down with your daughter and let her know the seriousness of this and that she will have to earn your trust again.