Debian Bullseye ships with the Linux Vendor Firmware Service (LVFS) fwupdmgr enabled by default.
There are many situations in which that’s a good thing; firmware is a central part of today’s hardware and software ecosystem, and you generally want to use the latest version available.
However, even though (supposedly; I haven’t yet been in a situation to actually experience this) actual updates need to be triggered manually, there are situations in which you want to reduce polling of external systems – especially when such polling could be used to deduce whether you have a particular piece of hardware or not.
Fortunately, it’s easy to disable the automatic checks in Debian. Simply enough:
sudo systemctl mask fwupd-refresh.timer
(For some reason, it is insufficient to simply disable the timer.)
You can still perform a manual check when appropriate by simply starting the unit that would normally be started by the timer:
sudo systemctl start fwupd-refresh.service
To see the result of the check, look at the unit log:
sudo journalctl --unit=fwupd-refresh.service