I don't think I remember the police ever knocking on my door
Anyway the point is that they don't just knock... they knock, then you answer, and then I assume they talk
Just "knocked on the door" implies they knocked and then ran, like naughty kids do
cops are never around when you need them, here. I wish they'd get rid of all these scumbag teenagers. but they're usually not openly breaking the law, so fuck all you can do