Despite having Global Entry, this has happened to me twice. I dread those four letters. The worst was in Barcelona and they took me to the basement in a small concrete block room. They left me in there for 10 minutes before finally coming in for a thorough check of me and my suitcase 🤨. Both times I was flying on two separate one way tickets (we had sailed Amsterdam to Barcelona in this case and one flight was booked with miles), and I'm convinced that's what caused it.