Re: mystery traffic
To be honest and I may be wrong, but it is possible someone cloned your modem's MAC address and is using it on a chipped modem.
If it is indeed the case, normally I would say get a replacement modem put in for yourself. Of course being registered blind it would be nice of them to get a technician to do it for you as the underside print is hard to see. But.. and a big but.. the 50mb modems no longer are available which would mean you would have to go on a superhub.
Most people don't like the superhub, but it also is a router built in, which comes with it's own firewall. Of course this can be turned off so it just acts as a modem (which I am led to believe from other posts from people who have one).
It is certainly worth trying since you're not a heavy user, and bad days for the customer support who told you to go for 50mb when you don't need it.