They can squeeze through an incredibly small gap. I had them get into my home when I was a kid several times. You'd hear it flapping around in the dark but when we turned on a light it would land. Then we held a box under it an knocked it down into it with a broom.

Funny thing, I was filling in for my wife for adoration this Thursday when a bat started to fly around inside the church. I opened a few windows in hopes he'd find his way out and tried to ignore him, but he kept making dive bombing passes at me. He finally decided to land way up by the ceiling over the choir loft. When the organist showed up to practice I told him about it and his response was "Again?"