Is my screen being shared?! With whom?
There may be a less drastic explanation for this. I had the exact same issue and found that this happens after using slack for calls where I share my screen. So just restart slack after your call has ended. Haven't tested it with other screen sharing software yet.

Some ones to watch are the obvious, Zoom, BlueJeans, Microsoft Teams, Google, as well as all manner of remote support apps that don't necessarily use Apple's Screen Sharing controls in system preferences.