Just a wild guess, but could it be a notification hidden by the 'Focus assist' feature? Go to Settings-System-Notifications&actions. Uncheck every tick box. Allow 'Get notifications' only from AutoPlay and Security&Maintenance. If you wish you can disable even those and disable all notifications at the top of the page. I like to leave the AutoPlay enabled since it tells when you can safely remove a USB stick.
Also, especially if you see Chrome and/or Edge in the show notifications list, go to the settings of those (for Chrome go to Advanced) -System: Disable running in the background when the browser is closed. Combined with notifications the worst I've seen happening is that every time an ad on some page changes a dark notification box bounces on the right - and most often the ads are about tits and/or gambling...