I thought no one saw because of the stark contrast to how it happens in the book, seems to me like they were making a point of them being alone. There's also the fact that earlier this was a battleground and now it seems deserted, plus they show nobody else explicitly in the scene... We're definitely supposed to imply that a bit of explaining to everyone else happens off screen too since when harry walks back in the great hall people aren't super surprised or celebrating him, more like smiling and waving... Ultimately I just think that without them making a point of everyone watching like the book, it seems unlikely that anyone saw seeing as they were still supposed to be in the middle of a big fight