Hi,
It depends what you have running on the computer, it could be normal or could be something malicious like a virus. Some things will run in the background and download updates themselves like anti-virus software or Windows Update (I can't remember if Apple update can be set to download without prompting but I think I downloaded 7 updates in the last month, you can check if that's downloaded anything this week in System Preferences -> Software Update -> Installed Updates).
Also chat programs and P2P if they are running can use bandwidth even if you aren't actually using them at the time. On the Mac you can see what's running with the Activity Monitor in Applications -> Utilities, the network graph will also show you the usage it's doing so you can see if it's constant 1 or 2KB/s or short spikes.
If your router is switched on even if the computer isn't then there will be a small amount of usage showing up, generally just a few KB at most but will include keep alive traffic from your router telling us it's still connected and random pings, port scans and probes and virus traffic that will just bounce off your firewall.
Dave Tomlinson
Enterprise Architect - Network & OSS
Plusnet Technology
