They are the common black house-fly.

I don't know how they got in; we don't keep our windows open... and in fact that window in the pantry doesn't have the option to open. But, we did have a bee in there last summer so there IS a way to get in!

How do I find the crack?

Is it likely that it's my dryer vent since that is my laundry room too?
It's right off the kitchen, pantry on one side, and room for the washer/dryer on the other.