Coincidental with this issue, I have had issues opening microsoft office files from my local machine. Occasionally there is an error that pops mentioning an issue with a dropbox add in. I went and found some dropbox integrated applications in the dropbox settings, and disabled all the microsoft office elements. I then rebooted, and my screen was free of the icon for a while, then it came on again for a minute, then it went away again. I will let you know if the problem persists. It had been there for about a week.
BTW - the icon is fixed over whatever is on screen - a browser window, an app window, a file explorer window, or even just over the desktop - so I doubt that using an incognito browser window would make any difference.
