Odd Message

[#1]

On a couple of occasions I have had a message box come up with the Plusnet logo & heading of Technicolor TG582n asking for a user name & Password.
I have no problems connecting to or using the internet & this message has just come up at random whilst viewing a website, it's  easy enough to get rid of but am just puzzled why it is appearing.
The router was set up as per the information provided with no problems but it would be interesting to know why this message has appeared.

[#2]

Do you have any browser tabs that automatically connect to the router? Do you have any quick-start plugins like speeddial for firefox that will periodically access the saved pages to update the thumbnail image.
The other option is something was trying to access the routers IP address via HTTP port 80.