What roarah wrote. Usually drain or sewer flies. Shower drains are common sites for habitation, sink drains less so.

They tend to lay eggs in partially clogged pipes, they like the held moisture. Flies usually come out at night, the larvae can be seen in and around the plumbing fixture in question. They might have come out of the drain and found a nice moist area in the track. Or if the track is moist and gunked up, it's possible eggs could have been laid there.

Easy way to diagnose is cover the shower drain with tape, sticky side down. Run lengths of tape across the drain with a small air gap between the lengths of tape. It might take a few days, but you may eventually see a fly stuck to the tape.

Remediation is to snake the line to remove any buildup that is holding moisture. Flush well. Then I recommend a scour with salt/baking soda/vinegar, let it bubble and sit, or use an enzyme-based drain cleaner. Flush well.

The goal is to scour the sides of the drain pipe clean to remove the moist gunk that they use for an egg laying site.

Tape the drain again and see if any more flies show.