We’ve experienced a constant presence of these guys in our apartment the last 3 months or so (see image).
We kill about 30-50 a day. At first I thought they were fruit flies (got rid of fruit lying out, they don’t seem to crowd around fruit though)…
…then thought they were drain flies (taped up drains, faucets, other plumbing spots to see where they are coming from, couldn’t find a source…also started religiously cleaning drains, looking out for our green garbage or any decomposing materials, dumping boiling water down each day, often cleaning with baking soda/salt/vinegar solution)…
…then thought they were fungus gnats (moved indoor plants outside, but we don’t see them crowding around those, we shake the plants and nothing comes out).
We don’t see a large number of them around any one spot, but they do irritate the hell out of us in our office rooms, in the bathroom and a small number can be seen dead in some of our windows when we open them. But so far no clear sign that they are coming from any particular spots.
It’s driving us nuts but from what I’ve read it’s not something pest control is likely to be able to solve permanently.