In some network environments where your HTTP(S) traffic gets routed through a firewall, you may notice in recent versions of Fedora that you occasionally get a big prompt open and display a web page or prompt for authentication.
After some packet capture, it appears that NetworkManager is doing some connectivity testing and attempts to pull https://fedoraproject.org/static/hotspot.txt
In some edge cases, where a firewall or other gateway needs authentication before processing requests, it will trigger GNOME’s captive portal detection and attempt to open what it thinks is a captive portal authentication page. Except that it’s not, and it will continually keep doing this every 5 minutes or so, or longer if you manage to get a session going with your firewall. This was noticed back in March this year when Fedora’s infrastructure team made some changes which caused it to trigger.
Essentially, the solution is to create an overriding connectivity check config in /etc/NetworkManager/conf.d/ where exists the default 20-connectivity-fedora.conf. Just copy it to 30-connectivity-fedora-disable.conf and remove the config values and then restart NetworkManager and that should disable it. Keep in mind, this might prevent GNOME’s captive portal detection from working, but it should still work if you open a browser.
This appears to also affect other distros using GNOME 3, however the above solution may not necessarily be correct for your distro and you may need to change configuration elsewhere.