IDG Answers is a community of experts who are passionate about technology. Ask a question or answer one below.
Also, I would change the SSID and password. On a side note, if you have kids and they have video game systems (console or handheld) or iPods, those are things that could account for some of the activity and be easy to forget about since they are not necessarily always online.
The first thing I would do is to enable MAC address filtering on your router, assuming that you already have WPA/WPA2 enabled (if you don't, you certainly should). You will have to get the MAC address for each device you want to allow, and enter it into the config screen for your router. This will only take a little effort on your part and should keep any rogue access points off your network. I’m not sure how to physically locate the unknown devices, but you could use something like Winfingerprint to determine the OS of the devices, so at least you know what you are looking for.