This would depend on what templates you are using, and how they are
configured. You can, if you wish, set custom iptables rules in sys-net
and sys-firewall to restrict traffic from those qubes, or deny it
completely.
At a minimum there is the clockVM which will look up time.
There is a function to check for updates - this is a service that you can
turn off. (Look in Global Settings in Qubes Manager.)
Fedora has (had?) a function to check for network connectivity.
Most of these can be removed/disabled.
You could run tcpdump in sys-net and capture the traffic if you want to
be absolutely certain of what is happening.