Bill walked into Ginny's room to look for her copy of Quidditch through the Ages. Ron was wrong, and he was going to prove it to the little bastard. Suddenly, he was hit with the sensation that it was too quiet in her room for it to be natural.
Bill worked through the facts simultaneously:
Snape and Ginny had been in this room, alone.
Snape was in love with her, by Bill's translation of the runes on her bracelet.
She'd looked flushed when she returned to the table.
In different clothes.
He tore the book apart with his bare hands.