If you're talking about the movement in the lower left of the screen, it's a strand of spider web. Maybe the "knock" you heard wasn't at the front door?

Hi @user52989. Like @WMRing mentioned, the movement in the bottom left of the video definitely looks like a spider web. That also likely triggered the motion detection initially, since it seems to be close to the Doorbell's lens. Brushing off any webs around your Doorbell should help. :)