I don't want to sound paranoid, but some people can be very determined. The subject came up on another forum I use, and this site was mentioned...
http://www.cantenna.com/
The freeware programme "Netstumbler" will also detect WiFi signals well beyond the useable range of the router, so I don't take
any chances!!
I believe the law now holds the registered user of an ISP responsible for any illegal use of his/her internet connection, so do you really want to take the risk?