If you want to pursue it further, the first thing I would do is remove the Cache/Website History on your Mac for that website. There is only one listed when you go to Safari > Settings > Privacy > Manage Website Data. The name of the file you can search for is flightradar24.com.
If that does not solve the problem, post the free EtreCheck report using the Additional Text option when posting. The report contains no personal information, and will show files that are launching on startup that may