what are Mrs. Van Daan's suspicions when she says "Ah hah!"? Do you think her suspicions are correct? Why?

Mrs. Van Daan expects to find the two in a compromising position...... turn back the clock to the 1940's, and yes, she was right.

[For a second ANNE stands looking up at PETER, longing for him to kiss her. As he makes no move, she turns away. Then suddenly PETER grabs her awkwardly in his arms, kissing her on the cheek. ANNE walks out dazed. She stands for a minute, her back to the people in the main room. As she regains her poise, she goes to her mother and father and MARGOT, silently kissing them. They murmur their good nights to her. As she is about to open her bedroom door, she catches sight of MRS. VAN DAAN. She goes quickly to her, taking her face in her hands and kissing her, first on one cheek and then on the other. Then she hurries off into her room. MRS. VAN DAAN looks after her and then looks over at PETER’s room. Her suspicions are confirmed.] 

Mrs. Van Daan (she knows). Ah hah!