Wikipedia is back up and running and gives this definition of a ghost sign:
A ghost sign is an old hand-painted advertising signage that has been preserved on a building for an extended period of time. The signage may be kept for its nostalgic appeal, or simply indifference by the owner.
I thank my friend, NY blogger Daryl for tipping me off to the term when she visited me here last summer. Now I'm on the prowl for more!