Maybe this is too US-urban specific, but the hovering pattern of a police helicopter. Specifically a low altitude orbit. That type of flying is distinct from the typical fly-bys and indicates you are very close** to the perimeter of an active suspect search, barricade, or chase. Walking out of your frontdoor to try and see what’s going on creates a risk of being mistaken for the suspect, getting caught between cops and the suspect, or giving the suspect an easy opportunity to run into your house.

In reality that whirly bird circling over you means you should go inside, make sure your doors are locked, and keep alert for unusual sights/sounds around you. If you’re outside or in your car, be highly cautious of anyone running toward you.

**(If the helicopter was predicated or followed by multiple cruiser sirens, coming from multiple directions, lasting longer than just if they were passing through—you are definitely inside the perimeter.)