Are you sure they aren't drain flies?
These are rather cunning little insects that live in your kitchen or bathroom drains and annoy the heck out of you. It's hard to catch them going in the drain, so don't poo-poo my theory yet.
Try and trap one, then show it to an exterminator. There are chemicals you can put in your sink to kill their eggs.
I also like the idea of checking the refrigerator drain pan. That would be my first action.
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