Do you have another Android device (like a phone, maybe) that you'd previously connected to that same wifiWi-Fi point?
Android 4, Ice Cream Sandwich, (and possibly older versions too, though I don't have any to hand to check) has an option to sync saved wifiWi-Fi passwords between different Android devices on the same Google account. Here's the option under Settings -> Backup & Reset on my phone.:
There are also apps that allow you to sync wifisync Wi-Fi passwords between your laptop and your Android devices, though I assume you'd know if you had one of those installed.