|Hopefully your Airport Express is configured to encrypt e.g. WPA or WPA2.|
In addition, if the IP address of your network is e.g. 192.168.x.x and you'll see an IP address of 10.0.1.4, you don't have to worry about it.
This could happen, if someone in your neigborhood also uses wireless network card.
The wireless network cards always looking around and searching for available wireless networks. In AdHoc mode, the card tries to connect to the wireless network with the best signal.
Please send a reply, if you solved the problem !!!