It is very easy for a bat to gain entry into a home. Bats only need 1/4 to 3/8 inch to squeeze into the attic space. This is about the size of the thickness of a pencle. When homes are built, there are ample gaps for bats to gain entry.
Here are some locations that are common bat entry points into a home: Eves, dormers, ridge vents, louvered vents, and under siding.