I find many of the ghost signs you see on here while riding my bike, or walking. This particular sign is on West McMicken, home to many great pieces of art, and a favorite route of mine since 2012.
I just found this sign in August, when my parents were visiting. I was driving them around, giving them a tour of the city, and I caught this out of the corner of my eye. I don't feel too badly about missing it before, because when I'm biking, I'm coming down the hill, and this is only visible going up. And, it hasn't been visible from Central Parkway until recently, because they just tore down some buildings and trees.
Note the secondary white square, which was probably another sign that has been covered up.