Perhaps there should be a case for physical memes. Ideas that have existed in the real world, that over time, get adopted for other purposes, often in the use of humor or political statement.
A quick search on the phrase 'In Case of Emergency' revealed quite a large number of offshoots from the original fire alert visual that we all know and have seen many times.
But where this simple communication construct has evolved is reminiscent of how we have seen the meme-ification of other transposed ideas spread across the internet.
There is the political/activist:
There is the advertising spin:
And like any good meme, you have the geek and zombie riffs:
There have been other physical memes before, but this is one of the most extensive I have found and look forward to where it brings us as the entire physical landscape becomes malleable as our digital world is.