Why on earth would the powers that be give any one any kind of warning by placing stickers on letter boxes.
you do know that they can see your house with google earth and just need to punch in co ordinates to mark people off on their computers.
Things like this would be done in secret, WWII was a different story because technology was no where near as advanced as it is now.
If they want to get rid of a lot of people in a hurry, they will just throw up Quarantine tape around properties and do it in the dark, no kicking in doors, or just have people go missing for no reason on their way to work.
Stickers on letterboxes are usually to help the mail or paper delivery person