The whole issue of Wireless security is a combination of Realty, Truth, Myth, and our love of Horror Movies.
If you Wireless Network is Not minimally Secured any one with a Wireless Client in their computer might stumble upon your network and use it.
If you use the security measures as mentioned above, people who are intend to do harm might find a way to log to the Network, and by intercepting the traffic work for Days (hours if they are lucky) and brake the encryption code.
You want to test? Take a Laptop go outside your house or (hallway if you are in an apartment) and see if you can log to your Network, pretending that you Do Not know your SSID (put in the SSID of the laptop WLAN which is the common default SSID). If you can learn how to intercept and break codes and see how long it takes to brake into your Network you would know how secure is your Wireless.
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