Just before Sarah accidentally changed the device's signal, some unknown idiot managed to get the signal playing over the school intercom system. Now, the signal was being heard all over the school. There was nowhere to hide from it, unless you just left.
The signal (the high-pitched noise that people hear when the device is used) is actually unique. Each personality option on the device's view screen menu has its own signal. But to the human ear, they all sound alike.
So, when the signal changed, Jon, Karyn, and the others didn't notice by hearing it. They noticed by seeing the effect.
