I also had an unpleasant experience with drain flies. Firstly, I didn't attract attention to them and I thought that they are harmless. But they began to appear so many of that I got worried. Indeed, we couldn't understand how do they get inside our bathroom and kitchen if we have insect screens on all the windows. I made inquiries on the Internet and it turned out that these guys live in sewers and water pipes. To tell the truth, it horrified me. So I learned how to fight with them and all the possible variants how to unclog a sink. So we got rid of them forever.