Your thinking that the fly traps are attracting flies is correct. They (flies) can smell them a long time before you can. However, they are highly effective. I have them inside the barn, but not where my fly predator larvae are spread each month..(.they have to be "cultivated" too). Moist ground, etc., and I have to put them in a place that the chickens can't get to...chickens will eat them! I have been very satisfied w/ the fly predators/ trap combination. With all the animals I have, I have to attack the fly problem on several different fronts. I keep Golden Marin Fly bait in the milk house, simply because it's cleaner, and I can keep it up high, so when I hose the milk house down morning and evening after milking the goats, I don't wash it away, I don't have to smell the traps, get sticky tape in my hair, and I don't have to swat at the predators! It is a constant battle, but the "three pronged" attack seems to keep the flies to a minimum.
I forgot to mention in an earlier post that I also encourage bats w/ bat houses (waaaay high on telephone/utility poles) and I have several martin houses that are all occupied as well, which helps keep down the mosquitos.