When flying back from Paris last year (CDG-ORD), I did online check in and had no checked bags so went straight to the gate. The person checking passports for the sticker at the gate seemed quite alarmed that I didn't have one, but then proceeded to ask me the standard questions (who packed your bag, has it been in your possession etc etc) and everything was fine. I wouldn't have thought it would be abnormal to not have talked to someone prior to that point, but apparently it was...