wireless wierdness

I have a Linksys Wireless Broadband router (WRT54G).

I have a Linksys Wireless USB network adapter (WUSB54G).

The adapter is located 15 feet from the router in clear line of sight.

The strength of the signal is 70%.

There is also a (unsecured) wireless network detected with a strength of 73%.

Where is this wireless network ? In the ceiling directly above my head ?