A friend of mine just had her home network hacked into by one of her neighbours this week. She thought the router had been set up properly but it obviously hadn’t been!
Many of the default settings on a router may not help to block out someone from getting in.
If you can see other networks on your own list of available network connections, then you are probably close enough for someone else to get access to your network.
Here is a great set of steps to check to see if someone is on your network:
Here is a great description to set up your router to make sure that it doesn’t happen to you.