Do you have a pest contractor? If not, get one, you need the expertise.
They will advise the correct location of an EFK which should be a "glue board" type. However, while this doesn't routinely eject dead flies, they can fall off, especially if the board is loaded. I would never put it above open product.
So, as others have said, why do you have the flies?
Let me tell you a story. In one manufacturer who shall remain nameless, I was reviewing the EFK counts. I thought "that's odd, there are loads of moths on that EFK but it's nowhere near a door that opens!"
I should have trusted the data. At 3am I once walked into the factory (the joys of insomnia). The fire door close to this EFK was open for ventilation. The moths should have given it away to me. Open door at night.
So the contractor should be helping you here but as I said, moths = open door. Tiny little flies which look like fruit flies? Dirty drains possibly with stagnant water in them. Flies which look a lot like house flies? Possibly cluster flies, get the contractor to look in any roof voids and treat up there.
If you need more support once you know what it is you're getting, pop back.