Offline bandwidth usuage

[#1]

I have recently returned from holiday ( 30th Nov.- 13 Dec.) and realising I had left the power on to my router but switched off my computer I checked the usage for the period expecting it to be zero on the days I was away.
However, I found that there has been a `Daily Allowance` usage of about 140KB and an `Overnight` usage of around 70KB each 24hours I was away.
How can a usage be recorded when my computer was off?
(The consistancy of the usage is within a couple of  KB each day and night and I have a fairly secure key/password so confident there is no hacking involved)

[#2]

HI pleasenlighten,
There will always be a small amount of usage as the router will communicate with the exchange to keep the connection alive.
Jojo

[#3]

Thanks Jojo

[#4]

There is also a lot of 'internet noise' which are the scanners/spammers/probes that try various random IP addresses trying to find a way into your network or router. Your router firewall will have blocked this as that is what it is there for. Some see several megabytes of usage due to this and it is perfectly normal.