So today I noticed a half dozen monster-sized flies in the kitchen and was wondering how they got there. I’m sure quite a few people here have experienced similar events.
Explanations? The most likely one I come up with is they flew in when the door was open.
Oh! Oh! I have one.
I once returned from a weekend away (Left Friday, returned Sunday) and found a few hundred flies on my kitchen windows. Not flying around, none in other rooms, all on those windows. No idea where they came from, never happened again. Was creepy as hell, though.
Perhaps you have fruit flies? The larvae might have been on some fruit or other food you brought into the house.
The good news is that they’re very easy to deal with. A cup of vinegar left out will trap many of them.
They might have spawned inside your house somewhere. I once found a pile of maggots underneath the bathroom sink. On a few other occasions, I came home and found the place full of flies, so I guess it’s happened a few other times too.
This was while I lived in a trailer out in a forest alongside a little creek. So in general there were flying and crawling bugs aplenty all the time (and lots of field mice too), but I mostly accepted that as something that just “comes with the territory” when you live out in the woods. There were lots of little green tree frogs too.
You probably brought the eggs in on some food. Enjoy!
We had flies hatch a inside a bag of mostly used up potting soil.
Not keeping that inside anymore.
You might have something dead in your house.
I once found a dead squirrel in the chimney. That’s when I remembered “Didn’t I have that problem with giant flies?” and “Remember when I was looking every where to find out what stunk?” Yet I don’t remember those things happening at the exact same time.
Kind of a sad and gross thing to find. But it did give me the chance to go on the roof to fix the screen on the chimney.
They deposit larvae in your ears overnight (the origin of the term “ear worm”). The larvae remain very still during your waking hours, then fly out at night as they mature.
Pretty sure.
I pity your younger brothers and sisters when they were children. Where did you tell them the bogeyman was hiding? Under the bed? In the closet?
We also returned from a weeklong vacation to find several giant flies circling an upstairs bathroom light that we had left on.
There was no odor of anything dead or rotting in the house, and we’re always careful to keep doors closed to keep flies out. If any get in, it’s just one or two average-sized houseflies at most.
But these suckers were huge, fat, slow flies. It was very creepy and gross.
Could be cluster flies. They tend to overwinter in houses and enter the house on warm days while trying to get back outside. Or, as other people have noted, you could have a dead mouse that’s returning to the ecosystem bit by bit.
Have you perhaps staked a severed pig’s head in the house?
Oh, yeah. Sometimes people forget to take those down after the holidays.
Don’t stop by just pitying my siblings, I get around a lot! 
Dead rat, most likely. We’ve had several threads about this. The only “good” thing one can say about the flies is that they are so slow, it’s easy to kill them.
The real question you should be asking is: How did a rat get inside my house (or inside the walls/attic/etc.).
I’m guessing flesh flies. And yes, as nasty as they sound.
Here is another contender, the blue bottle fly, as a half dozen doesn’t necessarily mean cluster flies. We get a few of these in the house every spring and fall.