It was just there. Flying around in circles, mostly. Right after dinner.
It flew around faster after we came into the room. We opened the doors but it didn't seem to notice them. We tried holding up a blanket to partition the room off smaller and move it toward a door, but that didn't seem to work either. Bats also don't seem to notice when you gesture and point and go "Look, a door!" So I suggested we stop moving and let the bat calm down. The circles slowed and got lower in the room, and then it flew out the patio door and away into the dusk.
Still a mystery.