Filth flies, including drain flies, are often seen around drains, toilets, and sinks that haven't been used for a few days. Organic material buildup (hair, scum, etc.) in drains, showers, and toilets is also a breeding site for these flies.
So what can you do if filth flies become a problem? The best solution for long-term control is to clean your drains. This includes removing and cleaning the traps under sinks. A plumber can help to ensure that organic material is not clogging traps in showers, bathtubs, and toilets.