This looks like a Sunflower Seed Maggot Fly (Neotephritis finalis), a native insect widespread in North America. The larvae feed in immature seeds of many plants in the aster family (sunflowers being one of them). If you have raw sunflower seeds that weren't heat-treated or otherwise processed to kill insects, or are saving native plant seeds for propagation, perhaps that's where the population is coming from. (We don't have a detailed list of this fly's host plants, but commonly-grown aster relatives include Eupatorium, Solidago, Liatris, Echinacea, Rudbeckia, and Helianthus.) These flies are not a typical household pest, so should not be a recurring problem if you can find the seeds they are breeding in and either remove them or seal their container.

Miri