I brought my personal notebook to work. Asked the IT technician to access the WiFi on it (it's a federal institution of health research, lots of biological risks involved). Then he noticed that the WiFi was already connected, but I never brought this notebook there before. My IT guy could not figure out how the laptop could have already been connected.
So my computer simply accessed a WPA2 WiFi network without knowing the proper password (and then it was memorized in the system, as I noticed later). Is that even possible? And how? It's a Linux Mint.