They might be drain fliesHi! Am battling the end of this problem myself. I thought they were fruit flies & I always just put a piece of fruit in a plastic bag, waited for them to go for it & then quickly closed the bag & took them out to the garbage. 2-3 bags & they were gone.
Well, this doesn't work w/these flies we have now. They don't care about fruit and nothing I did got rid of them. Then, I remembered something that my brother (who used to be an exterminator) said about drain flies.
So that's what they are but I wasn't willing to put the nasty chemical he recomended down my drains. And, they weren't breeding in the drains at first. I found the sources - which can be any organic matter. The main source was a few potatoes that had gone bad in my pantry behind some cans. That was beyond disgusting.
We also have a trash compactor so they were going in there too. So I started freezing any "live" garbage until I could take it down to the garbage. We also had some bad fruit so I bagged all fruit, onions, potatoes. They also like wet dishcloths so anytime I saw them on one I immediately put it through a hot wash.
For the drains I've used a natural drain enzyme cleaner found in health food stores. And no, bleach doesn't work according to my brother.
They will even get desperate & try to breed in those little training potties (ugh!) Dump & clean after every use.
We had swarms & there are only a few now. I think they are still in the drains & although the enzyme stuff is cleaning the drains I think it takes some time to get it all.