First off, she was supposed to be home. Wouldn't her parents have heard her leave the apartment? I have never lived in a quiet apartment. The walls are usually thin and sound travels. When my girls were 15, I KNEW if they were home or could hear if they left and that was in a 4 bedroom house.
I find it odd that her parents didn't check to see if she wanted to go to church, or check on her at all before they left. Strikes me as odd. Even if I knew my girls were asleep, I would check on them BEFORE I left and leave them a note! Odd.
Did they know she drank or used drugs at all? (The toxicology report). Were they used to her sneaking out to meet friends? Something with this whole scenario is strange.
If it was a "friend", why would she have had her pants and coat off. If they are SURE there was no foul play, but suspect that she will have something show up in a toxicology report, do they have information about parties that night or friends of hers that were seen nearby?
VERY strange.