How to stop and prevent continuous activity from a closed web page?

For the past few days, Screen Time has been showing me continuous activity from a specific website (I assume the grey circle is a website activity). The screenshot was taken this morning upon opening the lid of this M3 Max MacBook Pro which, at least in theory, should have been fully asleep all night. While this website _per se_ should be innocuous, I cannot find it anywhere among my open Safari windows and tabs. I've cleared history from Safari, quit Safari & waited to see what was happening and the activity kept rising. Activity Monitor doesn't show any entry connected to "Halstan.de" or to any part of it, in any tab (CPU, Energy, Memory, ...). What options do I have? Is it a bug in Screen Time that kept it stuck? The good thing is that I know I used that website, the bad thing is that the last tab from it was closed on Monday (24.06.2024). I've also added a time limit of one hour to just that site, but then opening a new tab and browsing to that same website works with no problem on all devices. Any help in understanding and possibly solving the issue is much appreciated.