Last week I came home on a sunny day to find the underside of the plastic carport roof swarming with flies, like regular house flies but smaller. Should I be looking for something dead nearby with maggot shells? This wasn't a normal amount, there were thousands, in fact I had to use another door to get in! Gross.
I've seen these in my garden too, they seem to be attracted to any warm surfice and they have normally gone by the following day.
I dont know what they are but the swarms seem harless. I think storm flies are smaller and bite.
I dont think you need to dig up the patio either!