I think you will find this is the background for the 3G signal bars - I believe they turn blue according to strength. According to the Extreme manual these become live when you use a 3G dongle. Here is a link to the manual. http://www.storageoptions.com/assets/do ... 0v.1.0.pdf
I haven't seen the manual enclosed with the Extreme as I haven't got an Extreme but is it is anything like the Excel postage stamp manual then you will need a magnifying glass to read it.
EDIT: I just had another look in the manual and it seems you may also be talking about your WIFI strength icon.