My husband is an entomologist and I can have him look later this morning! To my eye, it looks like any number of basic flies lol, not a pest per se but if you have a lot of them in your house there must be a food source for them somewhere, or something else attracting them in. I do not think they could come from drains. But like I said, it's my husband who is the professional so I will let him weigh in when he has a chance.

Thank you so much! This is the second round of them. The first time I got rid of them manually, and now a week or so later they’ve cropped up again. They must be laying eggs somewhere!

It looks like a house fly.

Sorry, my husband was busy all day and just had a chance to look. To identify the species he needs to know the size and have a clearer picture of the top of the fly. He said it doesn't look like a flesh eating fly (so probably not coming in due to a dead animal suck in a vent somewhere lol).

Other possibilities are flies that live on house plants, or cluster flies that come into houses to overwinter. If it's getting cooler where you live it's possible that's what's going on. But yeah, if you would like a definitive identification you can try taking a better picture and putting a coin or something to show the scale.