Philadelphia Ghost Signage Project

What is a ghost sign?
Also faded ad or brick ad. A faded, painted sign, at least 50 years old, on an exterior building wall heralding an obsolete product, an outdated trademark or a clue to the history of the building's occupancy. These signs often reappear after a rainstorm or following the demolition of a neighboring building.

See the Phila Ghost Sign Blog
And James Lileks Ghost Signs My introduction to them. ( His site is fun .. go play there)