In retrospect, Hermione knew that she shouldn't have been snooping around the headmasters office. She knew that she should have stayed in her sit with her face firmly forward and not noticing anything. But her curious mind wouldn't allow that.
What's that saying mum always uses? Curiosity killed the cat?
It was the curious mind that led her to turn her head to the side and to see the beautiful necklace. It was not her curious mind that led her to get up from her chair and to go to said beautiful necklace. No, that was her inner giddy girl. She had simply admired it for the first moment that she stood there, but then she had the overwhelming desire to touch it
Merlin, I wished I had squashed that desire.
That was how she ended up there, in the headmaster's office. In 1977.