They are most likely other gateways in your vicinity. Possibilities are a wireless router, a cell phone or iPad Wi-Fi etc. SecurityCenter labels them as intruders if they aren't known to it. Are you actually using a network? If not you can turn off such warnings b=y not trusting the network.
Open SecurityCenter by double-clicking the taskbar icon (or right-click > select SecurityCenter) then click PC and Home Network Tools
Then click Home Network Defense
Click on the router or network at the left of the diagram (see pic below - click to enlarge if nesessary) and select 'Don't trust Network' below.
Otherwise you have to keep on labelling anything it detects an intruder or not. Some like to do that and others don't, some have to because they use a network. But they only register as known if they are actually networked with that computer and you can recognize them. It isn't meant to replace the computer system's network settings, just gives you a quick reference to security that's all.