It seems as though wherever you go, you see "Free Public Wi-Fi”, an "Unsecured computer-to-computer” network. As far as I could tell, it’s completely useless, but maybe that’s because I have no idea how I could use it productively. So I decided to look into it, and here’s what I found out. The basic message is: don’t connect to these networks!
A "computer-to-computer” network allows two computers that have Wi-Fi to connect to each other without an external internet connection. In some cases, you expose yourself to risk by connecting to networks like these: it could be someone trying to get into your computer by promising you "free” wi-fi and hoping you’ll connect to their computer. In other (probably most) cases, these networks are other people’s computers sending out signals. In particular, if your computer can’t find a preferred network (and it’s running an older version of Windows XP), it will create an ad-hoc network with the same name as the last network it connected to, according to this article.
For interesting details about how these networks spread, see this article.