Traffic from Netgear modem - not connected to a computer

[#1]

We have a new PN service - that's been live since Thursday. Although computer is left off, we have left the Netgear 834PN router left running to "train" for a few days.
Rather surprising levels of traffic are reported on the PN
on Friday 714k and today so far 432k - both about 220 k between midnight and 8am.
The wireless network is screwed down tight security, and only people who know the passwords have not been near.
So what traffic would a router generate on it's own ?

[#2]

There is always a little background traffic between ISP and router - the amounts are generally measured in Kbs per day rather than anything significant.  It does not mean that anyone has accessed your router connection (the usage would have to be substantially higher for that). 
Whilst you should leave the router on most of the time during the "training period" it is usually recommended to turn it off occasionally (perhaps once a day) but leave it off for half an hour or so at least before turning back on so that the system does not mistake it for a crash.  If you don't use the computer at night then you could leave it turned off overnight? Turning off a bit and then on again encourages the line to re-synch and may help in getting a a steadier outcome from the training though it is better not to initiate a "turn on of the router" in the evening when general traffic is busiest and speeds are slowest

[#3]

You have an ip address.
Anyone can send traffic to that address and it "counts" whether you choose to let it through or not.
The level you are seeing is quite normal and fairly average.

[#4]

Thanks guys for the reassurance - I thought it was something of that nature.
Yes I will turn it off overnight as from Monday - when I'm next in the office.
being so close to the BT exchange does help