Fly Problem

We have been in our new house about 2 months. A few weeks ago we began to have a major fly problem in the basement, in the middle of winter. They were big ones too. I sprayed the entire basement and swept up about 30. This did not end it as dead and live ones were seen daily. We called the exterminator. They found a dead deer carcass in one of our window wells. Tjhis well was covered by a sheet of plexiglass that was all the way agaist the wall. I would love to know how a deer could hget down there and not move the plexiglass, and not break the basement window trying to get out. It was disgusting. luckily the PA game commission cameand got it. My thought is that someone put it there.