OK, I'll start... a friend had a roof leak a while back (blocked gutter I believe), and got some stains on the ceiling near his front door, but otherwise thought he got off lightly.
He rents out part of his basement, and asked his tenant to keep an eye out for water damage in that part of the basement. Tenant apparently said over and over that there was none
"Curiously, the same water leak that lead to the growth of the Panus also resulted in the growth of another mushroom immediately outside the structure growing on the outside of the home. This other organism’s fruiting body is known as a “brain fungus” and is probably either Lentinellus cochleatus or possibly a member of the Gyromitra. "
Are there are any sun roofs or vent pipes that you know of coming from the roof of this addition? Those are the most common places where the asphalt cement slowly washed away.
also make sure your flashing was done correctly too
you may have to walk on the roof a bit to know for sure