Dozens of motion notifications but video shows no movement

I get many motion notifications for my driveway cam every day, and the videos show no movement. I do have a couple garden flags in the front, but have set the motion zones so as not to include them. Further, I have the people motion setting on but, still, I keep getting notifications when there appears to be no movement. Do flag SHADOWS trip the motion camera? And even if it's set for people only mode? What can be the cause?