I kept many enclosures fairly humed and last summer I had a huge problem with intensely annoying tiny black flies zooming around the room.I let the substrate dry out and dont seem to see anything flying around in there anymore
I lost a sling to Phorid flies.Phorid flies are also called many other things since they show up on dead bodies and outhouses. They'll eat and breed on anything, and are a pain to get rid of if you don't keep things cleaned up. There are a few thousand species, and some are directly a threat and others are just annoying.
Also, you might google fungus gnats, they show up when conditions are wet enough, but are easily dealt with by drying out the enclosure.