Hi,
The problem with wireless routers is that other peoples machines will detect them if in range...
It's really not any different than going to a library or whatever with a WiFi hotspot.
It is entirely possible that your stuff is showing up on other networks as well...
That doesn't mean that they can actually connect....
There should be a way to specifically block the unknown devices, even though they may still be visible, they won't have access...
The security features you really need to investigate are most likely in your router, not your computer.
It is in fact possible that others are getting free internet through your router, but just because they show up, doesn't mean that they can actually access it.
In the past, there have been times when other networks appeared on my machine, but I was locked out from being able to access them...Of course, if the other access point doesn't have proper security, I have been able to connect through them..
I'm not an expert on this, but it is also possible that your wireless adapter in your computer is detecting the other devices simply because they are in range, but my experience is that it usually comes in through the router...