Have a look at the inSSIDer listing and look at the MAC address for your network, your guest network and the mystery network. If they are all transmitting from your CGN2 they will have the same or very similar MAC address. If you look at my images, and there are only 2, you can see a mystery network transmitted from my CGN3 after I decided to experiment with the USB stick that comes with the CGN3 and probably the CGN2 as well. Image #2 shows the mystery network at the very top, above the output power for the network that should be transmitting. Image #1 shows the same network on the very bottom of the list. Note that the MAC address is the same except for the last digit. In this case I managed to turn off the guest network but the mystery network remained. I was able to confirm that yes indeed, that mystery network did originate from my CGN3 by watching the inSSIDer display and physically turning off the modem, resulting in a gradual degradation of both the intended and unintended network transmission. That takes at least two to three minutes for inSSIDer to recalculate the numbers and drop the network from the display. Turn the power back on and yep, they both reappear. No amount of factory resets would solve the issue, as it appears that the parameters set by the USB stick are outside of the memory limits of the factory reset. Write off one CGN3. I'm wondering if you are in the same situation with your CGN2. Looking at the MAC address, watching the display while the signals die and are removed after unplugging the CGN2, and seeing them reappear when the CGN2 is powered up again will confirm that the mystery network belongs to your CGN2. It shouldn't be there, if it is. If it is transmitting, that means that whatever output power is going into that network is taken from the output power that should be used for the intended network.
The other explanation for the mystery network is a neighbouring router that is not transmitting its SSID. If you look at either image from my CGN3, you can see an example of that as well. The difference in this case is the MAC addresses which are physically different. Can you let us know what you find.
Can you also detail how you are transmitting a guest network. Is that via the Rogers USB stick, or from another router?