We use “ghost signs” for signs that no longer serve their original purpose, because the signified no longer exist. Typical examples are businesses that have closed or moved away. Such signs often show signs of decay, like fading or broken/missing letters. Also includes traces (ghost images) left by vanished signs.
Note that there’s a more prevalent alternative definition of the term, which limits it to “fading painted sign” [Roberts] or “old hand-painted advertising sign that has been preserved on a building for an extended period of time” [Wikipedia]. See the essay “What is a Ghost Sign?” (pdf) by Sam Roberts and Geraldine Marshall, included in Advertising and Public Memory (2017).