I'm running the free version of Dropbox in macOS Mojave.
When I pull up Network Activity in Norton Security Suite, I see 'dropbox.app' accessing the network seemingly every minute, even though nothing is active or has changed. (I use Dropbox in a very limited capacity.) This persists even when I pause syncing.
In contrast, OneDrive, which is installed next to Dropbox in the menu bar, doesn't do this, and seems to accesses the network only when a Onedrive file has changed.