Greta hadn't exactly figured out what she was going to do if she did find someone lurking in the broom shed. She wasn't too worried if it was a student, but hadn't there been talk of a blood-stained maniac running loose somewhere near the grounds? If there was one, they had better not be trying to steal one of her brooms, that was all.
The door to the shed was still closed and it didn't look like anyone had been obviously breaking in. That didn't mean anything though; it could be a tidy blood-stained maniac who didn't believe in excessive property damage.
She edged closer to the shed, holding her wand tightly in her hand. "Alohomora!" she whispered, and peered around the edge of the door as it opened, trying to get a look at whoever was inside without exposing herself too much.
When she did get a look, she almost dropped her wand in astonishment.