That's probably the worst (at least in the US) but far more likely is your 16 year old next door neighbour who finds he can get a perfect signal on your wireless network and uses it to download child porn or something like that. These things happen literally every day. I just struggle to understand why you would turn off security when you already had it enabled - unless you have an extremely fast WAN connection it really has no performance impact.

Just trying to make a helpful suggestion