Thanks for your follow up elizadoo.
It's common for CBP officers to question young female travelers about who they are traveling with and how they paid for their trip. Young women are sometimes duped into smuggling drugs by older men who offer them free trips and then hide drugs in their luggage or convince/intimidate them to carry back drugs. This is pretty much the plot of half the episodes of the TV show "Locked Up Abroad."
A CBP officer would think that most 23 year olds work, they don't travel to Istanbul, Latin America, and Europe for months juts living off savings and graduation gifts. I travel alone and I'm a frequent traveler and CBP officers often ask me "What do you do for a living?" When I say "lawyer" they promptly wave me through, because they conclude that lawyers have money and can travel the world without smuggling drugs. But if I were traveling back from Colombia and they asked what I do for a living and I said "Unemployed" or "I wash dishes at Applebee's" then they would probably detain me longer.
That said, I'm guessing that passengers lose claim checks all the time. If she has her boarding pass, or just regular ID, the airline should be able to determine that the bag is hers.
I guess this is a good lesson though to at least take a photo of your bag claim ticket in case you lose it or someone steals it.
I will add one final piece of info that others here have seen me write before. As a U.S. citizen, your daughter is not required to answer any questions from CBP officers about what she does in the U.S. or what she was doing outside the U.S. She is well within her rights to remain silent. If she's not smuggling contraband, CBP has no power over her other than the power to search her bags and person and let her go in a few hours.
Note: refusing to answer questions can cause them to very very aggressive and intimidating and I'm not necessarily recommending that a 23 year old woman do this unless she can handle the intimidation tactics. But it may give her comfort to know that despite their threats and asshat behavior, they can't lock her up for refusing to answer questions.