From the window where Satoru and the girl were positioned, they could clearly see the police cars silently parking outside. Just as he had advised, the police didn't use any sirens. Satoru actually wondered whether the authorities would be rational enough to not use any of their sirens but he didn't want to risk it. That's why he specified it in his message.
Now, it was time for phase two of Satoru's plan.
Without hesitation, Satoru lightly grabbed the girl and whispered to her to play along. Upon saying that, he regrouped with the kidnappers and told the girl to join the other kidnapped teenagers. Once again, the teenager who bullied Satoru eyed him suspiciously but he didn't say a single word since Satoru told him to shut up. Then again, they go to the same school so he could just ask him about it by then, assuming that he gets out of there alive, of course.