It might not be you. The S310 is a Wifi enabled printer. Since you can’t see the IP I’d guess someone near you owns one and your router can see it. Since it’s obviously not secured, you might want to tell him about it.
Me? I’d add the printer and print off some manuals about how to secure a wifi network, but I can be an ass about those things.
I know, your wifi is disabled. That means you are not transmitting, not that you can't recieve. All it takes to show a device is a brief contact while you where setting up your security.