He watches, and he notices things.
Professor McGonagall staring off into space during a lesson is one.
She doesn't do it too often, once every couple of lessons.
But then it turned into every lesson.
He watches her as she stares.
Every lesson he watches her stare blankly of into space for a few minutes.
For a while he assumed that she was staring at the wall, until Hermione had leant over and touched his arm to ask for more parchment, and he saw anger, jealousy and sadness flash across her delicate features.
He gives her the parchment and sees how McGonagall's hands relax once she takes her hand off his arm.
He notices, when she makes her rounds of the room, how close she stands to Hermione and how she touches her briefly in a congratulatory way.
He notices every time she does, the brief touches, linger.