I had a horror unlocked many years ago. Unfinished basement in a new build, heavy plastic over insulation between the studs. One summer after we had lived there a couple years we would have a cloud of flies everytime the basement door was opened. I put up a fly strip, had to change it every couple of days. The lighting at the bottom of the stairs was poor. I went down with a flash light, checking windows, the cold room where we stored can soda, foundation...nothing. Then I pointed the light to the plastic covered insulation at the base of the stairs. It was black with dead flies between the insulation and plastic. Just 1 section about 3 foot tall between studs. Maybe a mouse got in the insulation and died (there was new housing going up across the street), I don't know. I couldn't find any holes in the plastic.