Could have dropped the baby off at a hospital or fire station, no questions asked.

She's 15, she was probably terrified and scared. She may have also not know about that and I've heard that is actually untrue for some hospitals and fire stations.

Adults don't even make rational decisions when they are scared, I can't imagine someone that age would be any better.

Could have gone to a hospital and had the baby and immediately gave it up for adoption.

Again, she is 15.

And if she was giving birth at a friends house, there is a good chance her family didn't know she was pregnant or she didn't come from a family that could have helped her.