When Marinette starts receiving anonymous notes hinting at infidelity she thinks it's a joke - her kitty would never do that. However she was sick for a week. Nino was also sick at the same time. But, Alya and Adrien would never. The notes are incessant. The clues are becoming too hard to ignore. The question, could they actually have done it, was on Marinette's mind more often than not.