your hypothetical is always trotted out by every tyrannical leaning government, but never actually applies.
the government already has everything they need to surveil the population, arrest, detain, and search the population. if something gets through, either they fucked up and missed the opportunity to stop it, or there was no stopping it, period, and no amount of police state actions would've prevented it.
Agreed, it is more a hueristic or a feeling than anything factual. But I like that feeling. Can't help it.