Very interesting comments. I know that your first inclination would be that they are coming from the hoarders, but if you spoke with an entomogist, they would have told you that the drain flies live in drains. The bury their eggs in the built up soap scum and waste in your drainage system.
One thing you can try yourself is to take a baby bottle brush and clean your sink and tub drains thoroughly with a mixture of bleach and water. This may help to keep them from hanging around your unit. Another thing to try is to fill your sink with a small amount of bleach and hot water and do not plug it all the way. Allow the water to drain SLOWLY into the drain.
Hope they get the problem resolved!