A witch.

She could feel it. There was a witch at the school. Not a place Minerva could just ignore. While she wasn't certain where it was exactly near the school, she wasn't about to give up a Grief Seed, not when others could snatch it away from her. 

The motorcycle roared to life, before she sped off across the building tops. It was easy enough to mask her presence, even with her motorcycle at full speed jumping off building tops; engine noises weren't exactly foreign to anyone, and anyone who glimpsed her would just be told they were dreaming if they thought to mention it to anyone. That was, if they themselves didn't think they were dreaming. Finally reaching the road below, she put on her lights, speeding down the road at a reasonable speed. No sense getting a ticket from any night shift cops. Not when the storage compartment was stuffed with her guns and ammunition. 

Before long, she reached the school's front gate, locked as usual for this time of the day. Dissipating the motorcycle into mist, she jumped the fence, holding out her Soul Gem as she did so. Minerva smirked as she detected the witch's trail. Not a very smart witch, to make a labyrinth in this sort of place. After all, there were young girls there; prime candidates for Magical Girls. To make a labyrinth in a school like this was akin to a bird trying to nest inside an alligator's mouth.