Most likely refers to someone's wireless configured not to transmit the SSID.
Is my router broadcasting this BT Fon thing?
"As a member of BT Wi-fi, you share your wireless connection with other BT Wi-fi and FON members who are in range of your BT Home Hub. In return, you get access to broadband whenever you're in range of another member's Hub. You can also access Openzone and FON hotspots."