Trash-Stealer! Project Y

"There once was a girl barely anyone knew about. Every night, she would go from dorm to dorm, visiting every sleeping student. She wouldn't enter their rooms to hurt them, though. The only thing she would do is search through their trash cans, pick up a piece of trash and leave again. But if the student she is visiting isn't sleeping and she notices, he won't be able to move at all for some reason. It's like magic! And if he were to see her face, that student wouldn't see the one of a young, beautiful girl, but the one of a monster. There is something else to the story, though. If a student took something out of his trash can after already throwing it away, the girl would get angry and turn them into a monster. The student would never be seen again. And now, I can also assume that teachers staying at dormitories count, too."
Professor Aldric was surrounded by silence for a moment, then the class discussed his ridiculous story between each other.
"That's ridiculous," Ash muttered.
"So you're saying, Mr. Vale could've gone missing because he took trash out of his trash can?" a student asked, amused.
"The possibility is...," he paused for a moment, put seven fingers in the air and continued, "...at seven percent!"