Wow, this list is quite broad. I'm not sure if this is what you mean by "App to read secret physical secret messages that others have left, or id secret signs", but I would like to make an app that makes the real world like a scratchpad for people to leave messages and other people to read them. So, wherever you are, you just open the app and read messages that other people have left at that location or write your own. One implementation could be like a virtual graffiti app, but it doesn't necessarily have to be linked with physical structures. Not sure if something like this has already been done.