Do you have a woodstove?  Or a chimney with an open flue?  I used to get birds in the house in the summer through the chimney pipe of my woodstove in Virginia; it had a LONG pipe, about 15 feet, and was only about six inches diameter, but they would manage to get into the stove (making my cats crazy!  Bird TV Channel!).  The other chance might be an opening to the attic, like in a closet?  Or an opening in the wall, like where an outlet or switch plate might be missing?  Got me...but better you than me with this kind of visitor!