Just cos you can't receive your WiFi signal on your laptop when you walk down the garden/drive/to the edge of the estate....don't mean your signal is not there and is not easily accessible.
The man with a good antenna and software that is good at picking out each signal from the background noise will still be able to access the data from way beyond your perceived range of coverage.
The reference for this info is
this book
P.S. - I always ask people if they have a shredder at home when they talk about WiFi security at home, there is probably much more info which is more easily accessible in their bins. Maybe you all already have shredders, it's just a comment.
P.P.S. Don't buy the book in order to help you to find a home solution, it is very hard going and only of value to network professionals or those with network responsibilty (ok, ok, that means us too - we are responsible for our network at home, but you know what I mean).