An Outsiders Perspective
You see them again, stumbling out of a broom closet, after curfew, with their clothes rumpled, hair ruffled, lips swollen and you've no doubts about want they were doing just moments before. You see her bring out a pack of cigarettes put one between her lips, light it before passing the packet to him who does the same thing. You see them walking through a cloud of smoke but you're sure you saw them grasp hands. You go back to your dormitory but you don't forget what you saw.
You see them the next day at breakfast sitting opposite each other and occasionally you see them send smirks or wink at each other. You see him slide a slip of parchment across the table while continuing to talk to his friends. You see her reach out and grab the parchment before anyone notices, but you notice and you're fairly sure what was on that parchment. So when they stumbled into charms late with rumpled clothes and messed hair, you're definitely sure what was on that parchment.
You know they're not in a relationship but you also know when they're "together", they don't hook up with anyone else. You know this because you saw your dorm mate crying because he doesn't want to "have fun" anymore because he's with her. You also know that normally they would hook up with at least three people at one time but when they're together they don't. You realise that there's something more there than just two friends sleeping together.
You watch them closely the next few days and you notice the small details you ignored before, the small brushes, the reassuring hand squeezes, the lingering looks. You notice that any excuse for them to touch each other or just be alone together is used. You notice the looks she sends at him when she thinks no one is looking, the way he sketches her beautiful face on pieces of scrap parchment, and then tucks them in his pocket before anyone sees. But you see and you're certain what you see is love and you're also certain that they don't realise it yet.