It was standing on a trap door. Which means it wasn't there by accident. It's guarding something.
7 seconds sound clip from the Harry Potter and the Sorcerer's Stone movie soundboard.
You can hear this line at 01:06:40 in the Blu-ray version of the movie.
Quote context
[...]
- What do they think they're doing keeping a thing like that locked up in a school?
- You don't use your eyes, do you? Didn't you see what it was standing on?
- It was standing on a trap door. Which means it wasn't there by accident. It's guarding something.
- She needs to sort out her priorities.
- There are three kinds of balls. This one's called the Quaffle.
[...]