How are the Radleys and their home made to appear “mysterious”?

The Radley themselved were a mystery because they never came out. Boo was locked inside, Mr. Radley only emerged to buy groceries, and they held church services in their home. As for the house.....

The Radley Place jutted into a sharp curve beyond our house. Walking south, one faced its porch; the sidewalk turned and ran beside the lot. The house was low, was once white with a deep front porch and green shutters, but had long ago darkened to the color of the slate-gray yard around it. Rain-rotted shingles drooped over the eaves of the veranda; oak trees kept the sun away. The remains of a picket drunkenly guarded the front yard— a “swept” yard that was never swept— where johnson grass and rabbit-tobacco grew in abundance.