Last kiss goodbye in Paris

Sneak peek: Their last kiss in Paris is her most replayed memory, perhaps it was so heartbreaking for her because she knew it would be the last time she would see him. She leans down and kisses him softly on his lips. "I'll always love you, Jethro Gibbs." She says before turning away from the man she'll forever love. What happened the day of their last kiss in Paris?