Seriously, they’re attracted to the smell. Even if your garbage doesn’t smell to you, it’s attractive to the fries. Trash cans can become extremely smelly (and attractive) in the heat. A little moisture is all that will clinch it for them.

I don’t really understand the way you wrote the question. Do you mean to ask where they are before you put out some garbage? Or where they are before you open a bag of garbage?

I think @snowberry is correct in either case. They can smell stinky yummy garbage from a long ways away, and they can fly, so they may be scattered a hundred yards away and fly on over and concentrate where the best smells are.

Also, flies hatch from tinsy hard-to-see eggs laid in dead flesh, soil, fruit, etc., which can account for the sudden appearance of a bunch of flies even inside a garbage bag. Or a fly trap… one of the most horrible things I’ve ever seen was a fly trap which caught flies, who accumulated and died but not before they laid eggs that hatched into fly larvae etc…

where are you when the Burger King is closed?

I like this Q. I know when I mow the lawn or use my weed whacker along the tree line they get stirred up and fly around. I figure they hang out in the tall grass away from the sun and predatory birds. When they smell something delicious they head over to the buffet.

At someone else’s garbage.

Put a lid on your garbage can. Won’t stop the eggs that are already in there from hatching, but it will stop new comers from coming in.

I see. By the way If the dog poop has no flies around it what does it mean? Does it mean it’s too cold or something else?

Could be. If it’s warm enough, there will be flies.