We had a couple bats in our old house one time. The "bat guy" we called out found a huge infestation of bats in our attic. He said that when the weather gets really hot all of a sudden, the bats will move closer to cooler temperatures . . . as in the air conditioned interior of the house.
We lived in a 100+ year old house, in a neighborhood of 100+ year old houses. The bat guy said that just about every old building in the neighborhood probably had them. The owners just didn't know it. Once we knew what to look for we saw them everywhere. You'd see them come out and fly around the neighborhood each evening at dusk.
If you find yourself in a situation like this, the only thing you can do is seal up every tiny access to the attic/walls. They can slip in a crack as narrow as a quarter inch. Once the house is sealed up, you install a one way bat trap that they can go OUT of, but not back in.