Two busiest users of the Internet connection on my PCs, when I'm not streaming or actively browsing, are Windows Update and the antivirus/Internet security programs. Even when you decide on one AV product, others might install as part of a browser installation or update. In addition to Microsoft and your AV providers, the computer vendor could be running an update agent, and providers of add-ons (Java, anything from Adobe) are notorious for leaving update agents checking for and downloading updates.
There are also antivirus programs that go to a cloud service rather than running locally, or run locally but go to a server for signature data.
All of this without synching anything to a cloud data or backup server or a media sharing site. I'm surprised at how many applications have some agent talking to the Internet while I think the app is not running.
Laptops used to have a physical "network interface" (whether WiFi or wired) on-off switch, to help save battery power, but now you might have to do this from the control panel to disconnect.