Wireless networks are biggest victim of hacking and misuse of connection. Wireless network setup inside your home and office may be misused by your neighbors as a way to get free internet connection. You must protect your Wireless network with strong password to prevent unauthorized users from accessing.
However even after taking all precautionary measures to prevent unauthorized access one cant guarantee their wireless network will be safe. Here is one handy tool, Zamzom Wireless Network Tool which lets you to detect all computer names, Mac and IP addresses using your wireless network.