Understood, but a 400 yard diameter circle is a lot of real estate to secure.
I’d guess that you have read one of the many books about GySgt Carlos Hathcock.
And “unscheduled” events can be leaked.
Yep. Anything can be cracked or leaked. Reminds me of the saying, “3 people can keep a secret if 2 of them are dead”. And regarding the 400 yards, that would depend on the venue and other things. Some venues make it easier for screening, control etc. But nothing is perfect. No question.