Thank you all for you support & concern! My fiance's 15 year old cousin called a friend of hers who took down the caller ID information & turned it over to the police. She was picked up by authorities at a location in Indiana & placed in a juvenile holding cell until her Aunt got there to pick her up. She had met, and ran off with a guy she met on the Greyhound bus. They got off together in Chicago three weeks ago. We are happy she's safe at home.
What do you guys think is the best way to handle her homecoming?

I would show her that I am well concerned and also well fumed for all the worry she raised in the family due to her well-being and behavior.
This sort of behaviour is not acceptable.
She is only 15 anything could have happened to her, well anything can happen to anyone, so that why its good to know your loved ones whereabouts at least.

A thorough questioning on why she felt the need to do that?

I agree with needkarma too.

I agree with all 3 before me, but if it were my daughter I would also seek family counseling. Counseling may very well uncover some things that the parents can do better too.

... just my thought on it.