Thing is Andy......with no security key anyone can log into your network and access the internet, whilst the router is switched on, and it is unlikely that you would realise it, even if you were using it at the same time, just a minor degredation in upload/download speed.
An example.......I had to collect my wife from Downham Market railway station a fwe months ago.....i arrived fairly early, so sat and listened to the archers, then, being the nerdy geezer I am, got my laptop out and completed a couple of quotations i was preparing.......and lo and behold....4 wireless networks available.....3 unsecured!!!
I logged onto one, straight onto the internet.......I leave the rest to your imagination
Suffice to say I could have downloaded god knows what via that portal....and when the police call ...it's that poor blokes IP address that is flagged up as having downloaded the stuff.....
Its the wild west out there .... get your wireless keys set up for 128 bit, and make sure you have a working firewall