Their postures. Which had been... well, all over each other. So he knew that Anna had fallen in love quickly, but maaaaybe not that she'd wanted to marry Hans, just yet. Well then. Starting there and trying to save Anna's reputation...

"He is one of the princes, yes," Elsa replied, thinking. "They shared a private matter with me and it turned into an argument. Then, as you saw, she removed my glove and... thought I didn't care about her. And inside that palace... it would drive any extrovert mad. It's too quiet for her. I suggested she leave. And it only made things worse. Until...my ice just... I lost control."